Broken storage used to be used to test situations where the Git storage
wasn't being reached. These days we can just mock the Gitaly response.

But given the broken storage is removed now, Gitaly can take over
control of the storage being reachable. If it's not, Gitaly won't boot.
That's nice for situations where a disk wasn't mounted for instance.

Gitaly MR: https://gitlab.com/gitlab-org/gitaly/merge_requests/675