praefect/coordinator: Unify ErrRepositoryNotSet errors with Gitaly

The coordinator prefixes most of its errors with "repo scoped", which makes the errors mismatch those returned by Gitaly. Consequentially, Praefect does not act as an actually-transparent proxy. This can be painfully felt in our tests, where we need to distinguish between both errors.

Drop the prefix and return the error directly to unify the errors that both Gitaly and Praefect would return.