Remove backtraces from error messages

Backtraces are probably meaningless to most users and their presence generates a lot of email.

This replaces the error page with a generic message + a reference to the full logged trace in the case of a persistent message.
Hmm, those are indeed sub-optimal. Maybe then we can give them the 3 options displayed on home (Discuss, Develop, Translate) instead and cut out the middle step? I guess "going home" is the standard for 404's.