make "no searcher endpoints" error message more helpful

Created by: ijt

Before it wasn't clear what to do about the error.

Fixes #4101 (closed) in the sense that it will give users some idea what to do when they encounter that error. A better solution is incremental (a.k.a. "streaming") search #3991 (closed), but that is a larger project.

Test plan: Manually trigger #4101 (closed) again on k8s.sgdev.org by setting the search replicas back to 1 and issuing the query shown on that issue. Check that the error message looks reasonable.