I've finished testing handling errors in both types of detectors: reactors 
and auditors. Both seem to work fine.

The detector-error-catch.txt file contains changes to 3 files and was 
generated with this command:

$ hg diff roundup/cgi/client.py roundup/cgi/exceptions.py 
roundup/hyperdb.py > /tmp/detector-error-catch.txt

Hopefully it isn't too difficult to generate a patch file from this. Here 
is a summary of the changes. Comments welcome.

roundup/cgi/client.py
  Catches the DetectorError exception and sends email and generates HTML.
roundup/cgi/exceptions.py
  Defines the DetectorError exception class.
roundup/hyperdb.py
  Catches Exceptions from auditors or reactors and throws a DetectorError 
exception with the error details.