I came up with a better solution that I'm happy with. In the method 
fireReactors is a new try/except Exception block to catch detector 
execution errors. The except block creates a DetectorError with the 
error details (a new exception in roundup/cgi/exceptions.py). The big 
try/except block in Client.py's inner_main has a new except block to 
catch DetectorError and either write HTML to the user and send the 
traceback to the roundup admin, or write the traceback to the user.

This should be done with the method fireAuditors too. I'll work on that 
next (not much to do there) and post the hg diff file for the 3 files 
that were changed if that is OK with you.