AuthManager: Break AuthPlugin::addUser more explicitly

AuthManager: Break AuthPlugin::addUser more explicitly

AuthPlugin::addUser() is intended to only touch the external database
without creating a local user, which isn't possible under AuthManager
without reproducing too much of AuthManager's account creation methods
(and risking breaking things in even more obscure ways) to be
worthwhile.

As it is, either this will fail because the caller already called
User::addToDatabase() or the caller's subsequent User::addToDatabase()
call will fail because we're creating the local user.

So instead, let's just throw an exception unconditionally instead of
pretending it could work.

Bug: T137843
Change-Id: I8a439ea190c752a7fc49de5617e2c64c314c38f0