Add support for a credential denial error

[decentralgabe]

The spec currently does not handle the case where an issuer says "cool request bro, but no"

This text supports an issuer's right to deny a credential request for whatever reason, unrelated to the validity of the request itself

[Sakurann]

why would an issuer deny a credential request without giving the wallet any reason? does not sound like a good user experience.

[decentralgabe]

why would an issuer deny a credential request without giving the wallet any reason? does not sound like a good user experience.

Why wouldn't they give a reason? That's what the error_description field is for.