How does the 'app_not_configured_for_user' error differ from a '401 Unauthorized' error?

A '401 Unauthorized' error means you haven't provided any authentication credentials (or they are invalid). A '403 Forbidden' error, specifically 'app_not_configured_for_user', means you *have* authenticated, but the application doesn't recognize or allow your authenticated user access due to configuration issues. 401 is about *who* you are; 403 is about *what* you're allowed to do.