search: more consistently report timeout errors instead of "No results" when `timeout:` directive is present

Created by: ijt

This is progress on #3294 (closed) .

This also includes a helpful hint for a longer timeout directive in the error message.

Test plan: Manually tested with queries like func timeout:1ms on a sourcegraph instance with a single repo

Before:

After:

The commits shown on github are screwed up and include things from a previous PR. Sorry about that.