Fixed failed user authentication during GCM Registration

I've been working on an enhanced version of the BuildWatch app and noticed that when a user does not provide credentials (or wrong ones), a 500 Internal Server Error is thrown, with the stack trace and a message of "User not authenticated."  Apparently, the upstream code isn't handling your HttpResponses.error(403...) correctly.

So, I fixed the 500 Internal Server Error and changed the response from a 403 Forbidden to a 401 Unauthorized since it more accurately represents what's going on.  It also allows the Android HttpClient to automatically respond if you configure it with the Basic Auth credentials.