The unavailability of GitLab API makes us fallback to disk-serving

Currently, if the GitLab API is unavailable we fallback to using disk serving.

This is OK, now, but in a fully cloud-native architecture, this should rather return a proper 500 type of error.

We should disable local instance() and instead return a proper error to client.

func (d *Domain) resolve(r *http.Request) *serving.Request {
	request, _ := d.Resolver.Resolve(r)

	// TODO improve code around default serving, when `disk` serving gets removed
	// https://gitlab.com/gitlab-org/gitlab-pages/issues/353
	if request == nil {
		return &serving.Request{Serving: local.Instance()}
	}

	return request
}

// ServeFileHTTP returns true if something was served, false if not.
func (d *Domain) ServeFileHTTP(w http.ResponseWriter, r *http.Request) bool {
	if !d.HasLookupPath(r) {
		// TODO: this seems to be wrong: as we should rather return false, and
		// fallback to `ServeNotFoundHTTP` to handle this case
		httperrors.Serve404(w)
		return true
	}

	request := d.resolve(r)

	return request.ServeFileHTTP(w, r)
}