[Feature] 11253 copy project error messages always in english

[``* `#11253` Copy project always returns non-localized error messages``](https://www.openproject.org/work_packages/11253)
Why do you need this stub?

If you actually need it, it would be great if you could use a `Mail::Message` double as return, as that's what `ActionMailer::Base#mail` returns. In this case, the reason for having the stub apparently is not obvious, so I'd like a comment then.