misc: use minimal default 404 page when templating is down (#42162)

combo/public/templates/combo/native-404.html

@@ -0,0 +1,2 @@
+<h1>Not Found</h1>
+<p>The requested resource was not found on this server.</p>

combo/public/views.py

@@ -567,7 +567,7 @@ def error404(request, *args, **kwargs):
     if Page.objects.exists() and all((not x.is_visible(request.user) for x in Page.objects.filter(parent_id__isnull=True))):
         # if none of the first-level pages can be viewed by the user, display
         # native django error page.
-        return page_not_found(request, *args, **kwargs)
+        return page_not_found(request, *args, template_name='combo/native-404.html', **kwargs)
 
     try:
         page = Page.objects.get(slug='404')