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

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