Geo: An error in push to primary should tell user primary is down

If you bring down Unicorn on the primary and attempt to push on the secondary, the user sees:

$ git push
GitLab: GitLab is not responding (502)
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

We should give some indication to the user that the primary is down, not the secondary.