Sort out invitation failure responses #70
Description
An invitation can fail to validate in several ways. Currently we have:
- The invitation has been revoked
- The invitation cannot be used again
- The invitation has expired
- IDs don't match
- Signature provided is not valid
- This invitation code doesn't match
Need to think about how an application would deal with each of these scenarios. At any rate we should use codes in addition to user-facing messages so the app can respond in a structured way