Comment 6 for bug 264275

1) pass retcode=3 to your run_bzr call
2) New errors classes should have a unit test in test_errors, yes. bzrlib/tests/blackbox is the right place for tests of the command implementations in builtins.py, such as messages that are expected to be printed and how various command-line options are handled. (It is a bit confusing because that area also contains acceptance/integration tests of the full bzrlib stack of code). So if you've made the change in builtins.py, then a "blackbox" test or three is probably appropriate... but perhaps it would be better to put the fix in the core code (rather than in the command-line UI layer), and then unit test that?
3) Sorry, I haven't looked closely enough to have an opinion on this yet, but I hope my feedback on the other parts is helpful.

Thanks for doing this work!