You will have to do that in your application (for debugging purpose).
Python Example
In [1]: import requests
In [2]: r = requests.get("https://demo.erpnext.com/api/resource/test/error")
In [3]: r.text
Out[3]: u'<!DOCTYPE html>\n<!-- Built on Frappe. https://frappe.io/ -->\n<html lang="en">\n<head>\n\t<meta charset="utf-8">\n\t<meta name="viewport" content="width=device-width, initial-scale=1.0">\n\t<title>Server Error</title>\n\t<meta name="generator" content="frappe">\n\n \n\t<link rel="shortcut icon"\n\t\thref="/assets/erpnext/images/favicon.png"\n\t\ttype="image/x-icon">\n\t<link rel="icon"\n\t\thref="/assets/erpnext/images/favicon.png"\n\t\ttype="image/x-icon">\n \n\n\t\t\n\t\t<link type="text/css" rel="stylesheet" href="/assets/frappe/css/bootstrap.css">\n\t\t<link type="text/css" rel="stylesheet" href="/assets/css/frappe-web.css">\n\t\t<link type="text/css" rel="stylesheet" href="/assets/erpnext/css/website.css">\n\t\t<link type="text/css" rel="stylesheet" href="/website_theme.css"><script>\n\t\twindow.frappe = {};\n\t\tfrappe.ready_events = [];\n\t\tfrappe.ready = function(fn) {\n\t\t\tfrappe.ready_events.push(fn);\n\t\t}\n\t\twindow.dev_server = 0;\n </script>\n</head>\n<body data-path="api/resource/test/error">\n\t<div class="main-section">\n\t\t<div>\n\t\t\t<header>\n\t\t\t\t\t<nav class="navbar navbar-default navbar-main" role="navigation">\n\t\t<div class="container">\n\t\t\t<div class="navbar-header">\n\t\t\t\t<a class="navbar-brand ellipsis"\n\t\t\t\t\t href="/">\n\t\t\t\t\t<span>Home</span>\n\t\t\t\t</a>\n\t\t\t\t<div class="dropdown">\n\t\t\t\t\t<button class="btn btn-default navbar-toggle" type="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true">\n\t\t\t\t\t\t<i class="octicon octicon-three-bars"></i>\n\t\t\t\t\t</button>\n\t\t\t\t\t<ul class="dropdown-menu dropdown-menu-right">\n\n\n<li class="divider"></li>\n\n<!-- post login tools -->\n<li data-label="My Account" \n class=" logged-in" ><a href="/me" \n\trel="nofollow">\n\tMy Account\n\t</a></li><li data-label="Logout" \n class=" logged-in" ><a href="/?cmd=web_logout" \n\trel="nofollow">\n\tLogout\n\t</a></li>\n<li class="btn-login-area"><a href="/login">Login</a></li>\n\n\n</ul>\n\t\t\t\t</div>\n\t\t\t</div>\n\t\t\t<div class="hidden-xs">\n\t\t\t\t\n\t\t\t\t\n<ul class="nav navbar-nav navbar-right">\n\t\n\t\t\n\t<li class="shopping-cart hidden">\n\t\t<div class="cart-icon">\n\t\t\t<a class="dropdown-toggle" href="#" data-toggle="dropdown" id="navLogin">\n\t\t\t\tCart <span class="badge-wrapper" id="cart-count"></span>\n\t\t\t</a>\n\t\t\t<div id="cart-overlay" class="dropdown-menu shopping-cart-menu"></div>\n\t\t</div>\n\t </li>\n\n\t\n\t\n\t<!-- post login tools -->\n\n<li class="dropdown logged-in" id="website-post-login"\n\tdata-label="website-post-login" style="display: none">\n\t<a href="#" class="dropdown-toggle" data-toggle="dropdown">\n\t\t<span class="user-image-wrapper"></span>\n\t\t<span class="full-name"></span>\n\t\t<b class="caret"></b>\n\t</a>\n <ul class="dropdown-menu" role="menu"><li data-label="My Account" \n\t\t\t><a href="/me" \n\t\t\t\t\trel="nofollow">\n\t\t\t\t\tMy Account\n\t\t\t\t</a></li><li data-label="Logout" \n\t\t\t><a href="/?cmd=web_logout" \n\t\t\t\t\trel="nofollow">\n\t\t\t\t\tLogout\n\t\t\t\t</a></li></ul>\n</li>\n\n<li class="btn-login-area"><a href="/login">Login</a></li>\n\n\n</ul>\n\t\t\t\t\n\t\t\t</div>\n\t\t</div>\n\t</nav></header>\n\n <div class="hero-and-content">\n <div data-html-block="hero"></div>\n <div class="container">\n \n<div class="page-container" id="page-message" data-path="desk"\n\t>\n\t<div class="row ">\n\t\t\n\t\t<div class=" page-content col-sm-12 ">\n\t\t\t<div class="page-content-wrapper">\n\t\t\t\t<div class="row page-head">\n\t\t\t\t\t<div class=\'col-sm-12 hidden-xs\'>\n\t\t\t\t\t\t\n\t\t\t\t\t</div>\n\t\t\t\t\t<div class="col-sm-8 col-xs-6">\n\t\t\t\t\t\t\n\t\t\t\t\t</div>\n\t\t\t\t\t<div class="col-sm-4 col-xs-6">\n\t\t\t\t\t\t\n\n\t\t\t\t\t\t\n\t\t\t\t\t</div>\n\t\t\t\t</div>\n\t\t\t</div>\n\t\t\t<div class="page_content">\n\n<div class=\'page-card\'>\n\t<div class=\'page-card-head\'>\n\t\t<span class=\'indicator red\'>\n\t\t\tServer Error</span>\n\t</div>\n\t<p><pre>Traceback (most recent call last):\n File "/home/frappe/frappe-bench/apps/frappe/frappe/app.py", line 60, in application\n response = frappe.api.handle()\n File "/home/frappe/frappe-bench/apps/frappe/frappe/api.py", line 77, in handle\n doc = frappe.get_doc(doctype, name)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 606, in get_doc\n return frappe.model.document.get_doc(arg1, arg2)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 49, in get_doc\n controller = get_controller(doctype)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/model/base_document.py", line 35, in get_controller\n module = load_doctype_module(doctype, module_name)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/modules/utils.py", line 184, in load_doctype_module\n raise ImportError, \'Module import failed for {0} ({1})\'.format(doctype, module_name)\nImportError: Module import failed for test (frappe.core.doctype.test.test)\n</pre></p>\n\t\n\t<div><a href=\'/\' class=\'btn btn-primary btn-sm\'>\n\t\tHome</a></div>\n\t\n</div>\n\n<p class=\'text-muted text-center small\' style=\'margin-top: -20px;\'>Status: 500</p>\n\n<style>\n.hero-and-content {\n\tbackground-color: #f5f7fa;\n}\n\n</style>\n</div>\n\t\t</div>\n\t</div>\n</div>\n\n </div>\n </div></div>\n\t\t<div><footer class="web-footer">\n\t<section class="footer-links">\n\t\t<div class="container">\n\t\t\t<div class="row">\n\t\t\t\t<div class="col-sm-6 text-left">\n\t\t\t\t\t\n\t\t\t\t</div>\n\n\t\t\t\t<div class="col-sm-6 text-right">\n\t\t\t\t\t\n\t\t\t\t\t\t\n<div class=\'input-group input-group-sm pull-right footer-subscribe\'>\n\t<input class="form-control" type="text" id="footer-subscribe-email"\n\t\tplaceholder="Your email address...">\n\t<span class=\'input-group-btn\'>\n\t\t<button class="btn btn-default" type="button"\n\t\t\tid="footer-subscribe-button">Get Updates</button>\n\t</span>\n</div>\n\n<script>\nfrappe.ready(function() {\n\t$("#footer-subscribe-button").click(function() {\n\n\t\tif($("#footer-subscribe-email").val() && valid_email($("#footer-subscribe-email").val())) {\n\t\t\t$("#footer-subscribe-email").attr(\'disabled\', true);\n\t\t\t$("#footer-subscribe-button").html("Sending...")\n\t\t\t\t.attr("disabled", true);\n\t\t\terpnext.subscribe_to_newsletter({\n\t\t\t\temail: $("#footer-subscribe-email").val(),\n\t\t\t\tcallback: function(r) {\n\t\t\t\t\tif(!r.exc) {\n\t\t\t\t\t\t$("#footer-subscribe-button").html(__("Added"))\n\t\t\t\t\t\t\t.attr("disabled", true);\n\t\t\t\t\t} else {\n\t\t\t\t\t\t$("#footer-subscribe-button").html(__("Error: Not a valid id?"))\n\t\t\t\t\t\t\t.addClass("btn-danger").attr("disabled", false);\n\t\t\t\t\t\t$("#footer-subscribe-email").val("").attr(\'disabled\', false);\n\t\t\t\t\t}\n\t\t\t\t}\n\t\t\t});\n\t\t}\n\t\telse\n\t\t\tfrappe.msgprint(frappe._("Please enter valid email address"))\n\t});\n});\n</script>\n\n\n\t\t\t\t\t\n\t\t\t\t</div>\n\t\t\t</div>\n\t\t\t<div class="row footer-bottom-line">\n\t\t\t\t<div class="text-muted small col-sm-6 col-xs-12">\n\t\t\t\t\t\n\t\t\t\t\t\n\t\t\t\t</div>\n\t\t\t\t\n\t\t\t\t<div class="text-muted small col-sm-6 col-xs-12\n\t\t\t\t\ttext-right footer-powered">\n\t\t\t\t\t\n\t\t\t\t\t\t<a href="https://erpnext.com?source=website_footer" target="_blank" class="text-muted">\n\t\tPowered by ERPNext</a>\n\t\t\t\t\t\n\t\t\t\t</div>\n\t\t\t</div>\n\t\t</div>\n\t</section>\n</footer></div>\n\t</div>\n\t\n\t<!-- js should be loaded in body! -->\n\t<script type="text/javascript"\n\t\tsrc="/assets/frappe/js/lib/jquery/jquery.min.js"></script>\n\t<script type="text/javascript"\n\t\tsrc="/assets/js/frappe-web.min.js"></script>\n\t\n\t<script type="text/javascript" src="/website_script.js"></script>\n\t<script type="text/javascript" src="/assets/js/erpnext-web.min.js"></script>\n\n\t<script>frappe.csrf_token = "None";</script></body>\n</html>'
notice the error?
<pre>Traceback (most recent call last):\n File "/home/frappe/frappe-bench/apps/frappe/frappe/app.py", line 60, in application\n response = frappe.api.handle()\n File "/home/frappe/frappe-bench/apps/frappe/frappe/api.py", line 77, in handle\n doc = frappe.get_doc(doctype, name)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 606, in get_doc\n return frappe.model.document.get_doc(arg1, arg2)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 49, in get_doc\n controller = get_controller(doctype)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/model/base_document.py", line 35, in get_controller\n module = load_doctype_module(doctype, module_name)\n File "/home/frappe/frappe-bench/apps/frappe/frappe/modules/utils.py", line 184, in load_doctype_module\n raise ImportError, \'Module import failed for {0} ({1})\'.format(doctype, module_name)\nImportError: Module import failed for test (frappe.core.doctype.test.test)\n</pre>