addons-frontend: Pages stuck in "loading" mode when an error is thrown

When we use a loading flag in a reducer and the API layer throws an error (e.g. 400 or 404), then the redux state keeps loading set to true, which results in a page stuck in “loading” mode sent by the server.

For instance, here is a search result that throws a 400: https://addons.allizom.org/en-US/firefox/search/?platform=pouet&q=ad&type=extension. We do not get any error displayed and the page does not look nice. In #3268, I implemented a similar mechanism that should render a NotFound component when a 404 error is caught. Yet this does not work either.

Note: This does not seem to be the case when SSR is disabled.