Define error codes for the Credential Issuance Endpoint

The credential issuance endpoint has an error return body which is not thoroughly defined. Most importantly, the non-normative example in that section seems to suggest that the response should contain an “error” key, whose values are never explicitly defined.

This is in contrast to e.g. OAuth, which has an IANA registry for exactly this purpose, but more importantly, we need this defined here especially to support requesting a c_nonce, since the error code seems to be the issuer’s way of communicating to the client that it wishes to have a nonce used in the proof, and that it may try again using the given nonce.

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