Improve404 (#147)

* improve handling of error pages

the current version can handle files missing, but it can not handle
directories missing.

https://docs.stakater.com/mto/main/reference-guides/admin-clusterrole.html
Fails to show a nice looking 404.

https://docs.stakater.com/mto/main/obviously_missing_file
succeeds.

With this change, both should work.

Not sure what internal does or if is needed, have tested on a local
nginx only.

* needed for 403

I realized try_files failed to handle 403, so this repairs that part

* remove duplicate field

default.conf

@@ -2,6 +2,10 @@ server {
     listen 8080;
     root /usr/share/nginx/html/;
     index index.html;
+    error_page 403 /404.html;
+    location = /404.html {
+        internal;
+    }
     location / {
         try_files $uri $uri/ /404.html;
     }