Force OIDC error messages to use JSON
A lot of OIDC implementations omit Accept: application/json
when sending requests to the /token or /userinfo endpoint.
In case of an error, sendError is used, and returns an HTML page, which confuses most libs and makes errors harder to debug.
We should force errors encountered in Issuer::OpenIDConnect to use a JSON response type
See https://tools.ietf.org/html/rfc6749#section-5.2
The parameters are included in the entity-body of the HTTP response using the "application/json" media type