[Federation] error code registration

Issue #1512 resolved
Takahiko Kawasaki created an issue

Section 10.1.2 of OpenID Connect Federation 1.0 introduces two error codes, missing_trust_anchor and validation_failed, but Section 11 (IANA Considerations) does not have a section to register the error codes to the OAuth Extensions Error Registry. (cf. RFC 8628 Section 7.3)