Fix Incorrect Error Format in API for Timeouts

I found an area in the API where an error was being returned which did not follow the correct format of being marshalled into json with a 'Message' field. I fixed this, and a test that broke after fixing this.

There is a question of compatibility, but our API is pretty specific about how errors should be presented and handled, and this was in violation of that spec. I don't think it should be considered a compatibility break to change this, it's a bugfix and hopefully anyone who ran into it would have informed us about the inconsistency rather than just working around it as a special case for timeout errors.