... while raising Reject doesn't.
See the following test-action. Put into your tracker
extensions directory and then try the following two urls:
http://tracker-path/issue?@action=testaction&exception=reject
http://tracker-path/issue?@action=testaction&exception=valueerror

when raising ValueError, a nice error message is
printed, while for reject a backtrace results.

I think for reject a nice message should be printed, too?

this happens with 0.8.3.