searcher: do not log net.OpError

Created by: keegancsmith

I was watching searcher logs and noticed huge amounts of log spam when a connection is closed. This is since we can try to find several matches before we check if context is canceled. We then get a log message per match informing us of something like:

write tcp 10.164.18.30:3181->10.164.27.49:56316: write: broken pipe

Test Plan: To reproduce I made hybrid search in zoekt write ^10 more matches, then added some timing which called ts.Config.Close(). This reproduced the error. Additionally I think the error checking code is a little tricky, so I introduced a test specifically for the helper.

Part of https://github.com/sourcegraph/sourcegraph/issues/37112