web: Handle missing partial graphql response when repo-updater is down
Created by: keegancsmith
Previously we would fail later in our app with "Cannot read property nodes of null". This check now passes on the actual error.
Test Plan: Killed repo-updater and observing an error about failing to contact repo-updater on the settings page.
Fixes https://github.com/sourcegraph/sourcegraph/issues/3593