Add standard error handling path

[JD-P]

Right now it's unclear to clients (e.g, GreaterWrong) what should be done in response to an error code from the server. It would be helpful if error responses included json payloads with more information such as whether the error is recoverable and what error message should be shown to the user. This could then be used by clients to automatically distinguish between user-recoverable errors and errors which are not recoverable.