Dashboard::ProjectsController should tolerate single Gitaly node failure

This issue is one piece of #33616 (closed), which was part of a production incident.

---

If a single Gitaly node fails in some critical way (cannot be reset, rebooted, removed, etc), and you have repositories on that server, you will experience 500 errors when rendering bits of pages that need access to that data.

This controller includes /dashboard/projects and /dashboard/projects/starred.

How would this look?

Example of when a TODO (realize this is not the right controller) would fail to render because of this issue, and how we would render alternative content:

/cc @andrewn