Made Ganymede react properly if a logged-in user or persona is deleted

If a user or admin persona is logged into Ganymede and another session
deletes that user or admin persona, the session whose rug has been pulled
will be aborted on the next permissions check, or when the transaction
attempts to commit.
Added an exception-wrapping constructor for NotLoggedInException
Made DBPermissionManager respond more gracefully to a deleted user/persona