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

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