Clarify how deferred credential response indicates the issuer no longer plans to issue a credential

[jogu]

The response can also indicate that there are no credentials for the holder (for a variety of reasons) and that the wallet should not poll any longer. I think it's worth adding normative description of what this response looks like. One of the following probably:

• 204
• 200 with an empty response
• 200 & and empty credentials array
• 400 & error credential_request_denied or a new error code such as no_deferred_credential

Originally posted by @sloops77 in #492 (comment)

[jogu]

Discussed on today's WG call:

An explicit error to tell the Wallet that the Issuer no longer intends to issue a credential would be good - we can use 400 error with existing "credential_request_denied" error to mean that a credential will never be issued under the used transaction_id.