Use standard error response for failed mTLS auth

[josephdecock]

When the mtls middleware fails to validate the incoming client
certificate, or if no certificate is presented, it now returns an HTTP
400 response, and includes the standardized json error:

{
   "error": "invalid_client",
   "error_description": "mTLS authentication failed"
 }

This conforms to RFC 8705:

If no certificate is presented, or that which is presented doesn't
match that which is expected for the given client_id, the
authorization server returns a normal OAuth 2.0 error response per
Section 5.2 of [RFC6749] with the invalid_client error code to
indicate failed client authentication.