Ensure user callbacks exceptions are propagated to the user on consumer poll

When user-defined callbacks fail with an exception, the expectation is that the error should be propagated to the user as a KafkaExpception and break the poll loop (behaviour in the legacy coordinator). The new consumer executes callbacks in the application thread, and sends an event to the background with the callback result and error if any, passing the error along with the event here to the background thread, but does not seem to propagate the exception to the user.