Silently doesn't create menu links if permissions don't allow

Takes a long time to debug if the permissions are not correct as there is no report as to why the relationship is not created.

The attached patch just creates a watchdog message as we use nodehierarchy in the background. i.e. hide the form element and programmatically map child documents to parents rather than making it a user-driven choice. As such it seemed more appropriate to throw an exception or generate a watchdog message than to create a status message.