Render gitaly-unavailable error for Tags page

What does this MR do and why?

Related issue: #340818 (closed)

If Gitaly is not available, we should robustly handle the error and display a sensible message to a user:

Before: 500 error was rendered

After:

How to set up and validate locally

1. Go to project page
2. Click on Branches link (should be something like https://gitlab.com/gitlab-org/gitlab/-/branches)
3. Stop gitaly process

gdk stop gitaly

4. Refresh the page
5. Verify that page successfully loads and shows an error message