Make app error page customizable

src/nginxconfig.ejs

@@ -128,7 +128,14 @@ server {
     # only serve up the status page if we get proxy gateway errors
     root <%= sourceDir %>/dashboard/dist;
     # some apps use 503 to indicate updating or maintenance
-    error_page 502 504 /appstatus.html;
+    error_page 502 504 /app_error_page;
+    location /app_error_page {
+        root /home/yellowtent/boxdata;
+        # the first argument looks for file under the root
+        try_files /app_not_responding.html /appstatus.html;
+        # internal means this is for internal routing and cannot be accessed as URL from browser
+        internal;
+    }
     location /appstatus.html {
         internal;
     }