Backport #23387Close#22934
In `/user/repos` API (and other APIs related to creating repos), user
can specify a readme template for auto init. At present, if the
specified template does not exist, a `500` will be returned . This PR
improved the logic and will return a `400` instead of `500`.
Co-authored-by: techknowlogick <techknowlogick@gitea.io>