Ability to get error by sequence number or a custom format.

[GoogleCodeExporter]

We sometimes use the unique identifier on the custom error page to identify a 
specific error.  It is awkward for users to give these to us over the phone 
when they experience an error.  Ideally it would be better to retrive an error 
by sequence number as it's an integer, even better would be another field for a 
custom error format.  I appreciate this is only recorded in certain log 
formats, and a custom error identifier format doesn't exist.

Original issue reported on code.google.com by [email protected] on 13 May 2014 at 2:38