Return fetch errors from FetchInternalRemote

For historical reasons FetchInternalRemote returns a boolean to indicate if the fetch succeeded or not. The only caller of FetchInternalRemote is ReplicateRepository. To make this call easier to diagnose we should return the errors directly.

This is strictly speaking a breaking change, so I'm hiding it behind a feature flag.

gitlab-rails no longer calls FetchInternalRemote gitlab@8e699fe9