Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/463022

**Problem**

`Gitlab::GitAccessProject::CreationError` is not handled by
`internal/allowed` endpoint. When the project fails to create, then we
return `Internal API unreachable` error.

**Solution**

Add a handler for `Gitlab::GitAccessProject::CreationError` and return
an error message.

Changelog: fixed