Sorry, I missed to refer nginx error log… seems like some issue with default path generated by nginx setup. Error log helped me to correct the path and now nginx restarted.
ubuntu@ip-172-30-0-240:/var/log/nginx$ tail -f error.log
2015/04/16 11:28:31 [emerg] 16602#0: unknown directive "rewrite^https://www.nescode.com" in /etc/nginx/conf.d/frappe.conf:61
2015/04/16 11:34:12 [emerg] 16732#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:38:50 [emerg] 16775#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:41:20 [emerg] 16821#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:43:41 [emerg] 16859#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:44:49 [emerg] 1939#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:47:42 [emerg] 2461#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:55:29 [emerg] 2529#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 11:56:03 [emerg] 2561#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
2015/04/16 12:15:15 [emerg] 2628#0: BIO_new_file("/etc/nginx/ssl/certificate.crt") failed (SSL: error:02001002:system library:fopen:No such file or directory:fopen('/etc/nginx/ssl/certificate.crt','r') error:2006D080:BIO routines:BIO_new_file:no such file)
^C