Clarify error handling

[philipashlock]

While a client can determine the required fields from the service definition, the possible errors in the v2 spec from posting a service request doesn't account for how the client should interpret the errors from unfulfilled requirements, malformed values, etc. In general, the spec should define error response data structures as first class specifications as it does with normal responses.

http://lists.open311.org/r/post/15QBKcwHYe66W5YbLfSkfD

http://lists.open311.org/groups/discuss/messages/topic/1ZQVCqMre9ngZvMMoDOnh1