1. We have no formal mechanism for informing the client when unexpected exceptions happen (as opposed to validation failures).
2. The general scheme should follow https://wave.google.com/wave/waveref/googlewave.com/w+UnRfCRd2C
(specifically, ConstraintViolation should not be part of SyncResults. It should be a separate field in the response, not part of sideEffects. All Receivers need to be updated.)