Improved error messaging on failed authentication with Client Identifier

[jeswr]

Feature description

It would be useful to have better error messaging around some cases when authentication fails due to the contents of the Client Identifier Document. The particular error that I found confusing was when I tried (incorrectly) to use a the following Client Id Document containing relative URIs:

{
  "@context": ["https://www.w3.org/ns/solid/oidc-context.jsonld"],
  "client_id": "",
  "client_name": "Comunica Web Client",
  "redirect_uris": ["/"],
  "post_logout_redirect_uris": ["/"],
  "client_uri": "/",
  "logo_uri" : "/images/logo.svg",
  "scope" : "openid profile offline_access webid",
  "grant_types" : ["refresh_token","authorization_code"],
  "response_types" : ["code"],
  "default_max_age" : 3600,
  "require_auth_time" : true
}

The error message I receive is "The client registration client_id field must match the client WebID"; which is a little confusing when they do match so far as the json-ld spec is concerned. For those kinds if error messages I think an isValidUri check should be applied first and the error message should indicate whether the client_id field contains an absolute uri and note that one is required.

This should be done for all other fields that required absolute URIs (which I believe are all of them so far as the spec is concerned).

[joachimvh]

Sure we could add an extra check to specific fields in the WebIdAdapterFactory (and also rename that class to something that makes more sense).