Detect GitHub 404 errors during background permissions syncing

Created by: unknwon

During background permissions syncing, 404 usually means 403 for GitHub. We need to detect and swallow such errors so we don't spam logs with "404 not found".

15:43:32 repo-updater | WARN Failed to sync permissions, type: 1, id: 1428, err: fetch repository permissions: request to http://127.0.0.1:3180/repos/sourcegraph/<REDACTED>/collaborators?&page=1&per_page=100 returned status 404: Not Found