Make error reporting more verbose.

Based on $wgShowExceptionDetails, MWException outputs a stack trace. Our own
implementation omitted this.

It should now still add our own (more user-friendly) error message, but also
call MWException's getHTML, which may output a stack trace too.

Change-Id: Iceb03ea22080903cd4f9897a9bf77428a6100c8e
Merge "Make error reporting more verbose."