Error message pages on the server.

[ogoffart]

This is a spin off of https://github.com/owncloud/oauth2/issues/34 which added already a URL where the client redirects when the authorisation is successfull.

But it would also be nice to have url messages like that when the authorization is not successful. (i.e, any condition for which the token endpoint may return an error. The server could just return another message_url.

Error may be:

• The client secret is wrong
• The code= is invalid
• Anything else that can cause the grant_type=authorization_code to fail.

Currently, the client show a very basic non-styled html page, and it would be nice to show a proper themed page. But since error condition normally do not appears in a typical workflow (these are basically server of client configuration errors), this issue is not so important.