Add support for specifying HTTP status codes on errors.
Review Request #1366 — Created Jan. 28, 2010 and submitted
Add support for specifying HTTP status codes on errors. This change introduces proper HTTP status codes for the various errors we send out. This will break backwards-compatibility if a client assumes that every error type is going to be a 200 OK. It's a bad assumption in general, but sadly, is made in post-review (which will soon be fixed).