invalid_redirect_uri error raised by POST /oauth2/registration doesn't explain which policy was violated

*This issue was originally created by [**@Half-Shot**](https://github.com/Half-Shot) at .*

It wasn't clear to me that I couldn't set a localhost URL for the registration request URIs because it's forbidden in https://github.com/matrix-org/matrix-authentication-service/blob/07c9989e63048ace17d00c1068b2c456651d0b61/crates/handlers/src/oauth2/registration.rs#L191-L243

The error is super vague though, and could do with some explanation (or at least a log line).