Show custom error for ingress-generated pages

We have nice custom Stackspin error pages, which work perfectly for example when a service is down (503). We tried using them for 404 and 500 errors as well, but then they work a little too well, also replacing 404 and 500 error pages from apps. We'd like to serve the custom error pages only for errors that are generated by the ingress controller, not for ones that are served by the upstream apps.

See for example this: https://github.com/kubernetes/ingress-nginx/issues/8376