Improve error message when topology service is down

The following discussion from !3820 (merged) should be addressed:

• @kkloss started a discussion:

  suggestion (non-blocking): when the topology service is not up (be it because it had a slow start or because of an error), the HTTP router will show a “Network connection lost.” error.

  It’s fine for now but let’s improve the error message in a follow-up to something like “Could not connect to the topology service. Check that it is running, for example try gdk status gitlab-topology-service.”

Proposal

by @OmarQunsulGitlab

@bmarjanovic maybe this can be done via gdk doctor, by adding a new Ruby class here: https://gitlab.com/gitlab-org/gitlab-development-kit/-/tree/main/lib/gdk/diagnostic?ref_type=heads to check that Topology Service is working, if it was meant to be enabled. I think that's the proper way to do it. It can do that by calling the List Cells endpoint /v1/cells, for example.