Extensions that create an AuthBackend subclass (using the AuthBackendHook) can throw exceptions inside Reviewboard. The parts of Reviewboard that call those methods have been sandboxed.
Now when an AuthBackend subclass from an extension throws an exception; Reviewboard logs the errors with enough information to find which method in the AuthBackend subclass threw the exception.

Unit tests have been written to make sure that functions from an AuthBackend subclass have been called, and when an exception is thrown it gets logged.
The tests fail without the sanboxing, and succeed with it.