Comment 3 for bug 504070

Fixing this isn't as simple as adapting to the testtools api changes unfortunately. The way testtools runs cases differs significantly from how unittest does it, and addError/addFailure are called outside the exception context, after all the cleanups have run, so pdb has no traceback to be looking at.

I'll fix the bzr side of this, making testtools not break the expectation we can get the right stack from sys.exc_info is harder.