No way to raise authorization errors when doing validation

[espenhogbakk]

I'm trying to implement this according to standards, and when looking at your example you mention that it would be a good idea to validate client_id, scope and expiration. I'm trying to do that in my implementation, but I can't seem to find a good way to return an error in the exchange.

How do you do return done(some error descriping what is wrong))?