s/caught/uncaught/ since this is library code, we want to message that the handler method threw an exception and it was not caught by that code? (I don't have a strong feeling about this)

It might be good to say `[%s:%s] uncaught exception. requeueing  message %r` or in the case of already responded to explicitly say, `uncaught exception. message already responded to`.

Since we don't log every message handled, it could be useful to log it in the exception case.