access_denied
According to [RFC 6749](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2.1), when the resource owner or authorization server denied an request, an `access_denied` error should be returned. But currently in this case Gitea does not return any error. For example, if the user clicks "Cancel" here, an `access_denied` error should be returned. <img width="360px" src="https://github.com/go-gitea/gitea/assets/15528715/be31c09b-4c0a-4701-b7a4-f54b8fe3a6c5" /> (cherry picked from commit f1d9f18d96050d89a4085c961f572f07b1e653d1)