Fix error handling when posting a review request.

If an unhandled error code was returned from the server, the process
would just quit without any useful indication of what went wrong. We now
use the error message from the server.

On top of this, rbt-post and post-review were checking against the wrong
error code for the Empty Diff error.

Tested with an unhandled error code (by removing the default handling
temporarily).

Tested an empty diff.