In the PageGenerator, if a ProcessingException is thrown by a zone item (mostly services), it's caught and generated in the XML as an error.
If a service throws an AuthorizationRequiredException or an AccessDeniedException, it's wrapped in a ProcessingException and caught by the preceding behavior, whereas we'd like the exception to be thrown (resulting in a 401 or 403 response code being sent to the user), instead of being generated in the page.