frontend: Fallback to generic not found error page

Created by: keegancsmith

This was accidentally changed in a refactoring. Previously we would only check errcode.IsNotFound once doing other more specific not found error handling. However, the check was moved earlier into our ui handler leading to repositories that are cloning to have a 404 response (instead of 200).

Test plan: Locally by visiting github.com/sourcegraphtest/AlwaysCloningTest and added unit tests

Fixes https://github.com/sourcegraph/sourcegraph/issues/4081