Draft: Enable gitaly_fetch_internal_remote_errors by default

#3588 (closed)

Since FetchInternalRemote is only used by ReplicateRepository we can't do a normal feature flag rollout for this feature. Instead we're going to try enable it by default with an environment variable to disable it again in an emergency.

Setting GITALY_DISABLE_FETCH_INTERNAL_REMOTE_ERRORS to 1 will disable the feature.

Keeping this MR as draft so that it doesn't get merged by accident as the release will need to be coordinated.