[Geo] Don't mark sync as successful if repo does not exist because of some problems
The issue based on the @brodock's suggestion, see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4677#note_60650064
A secondary node marks every sync as successful if the repository does not exist. While it's OK for the majority of the cases it does not take into account some possible problems with NFS servers availability.
To make more precise decisions we could make use of repository checksum. If one exists but the repo doesn't, it's an indicator that something is wrong with the infrastructure.
Blocked by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4749
/cc @mkozono