Sort out invitation failure responses

[HerbCaudill]

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• The invitation has been revoked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