fix: say the server is down and keep retrying instead of a database error (#1637)

docker/nginx-https.conf

@@ -476,9 +476,12 @@ http {
         }
 
         error_page 500 502 503 504 /50x.html;
+        # Only nginx's own errors land here (the backend is down or too slow).
+        # There is no 50x page, so answer directly instead of a 404.
         location = /50x.html {
-            root /app/html;
             internal;
+            default_type application/json;
+            return 502 '{"error":"The Termix server is not responding"}';
         }
     }
 }

docker/nginx.conf

@@ -445,9 +445,12 @@ http {
         }
 
         error_page 500 502 503 504 /50x.html;
+        # Only nginx's own errors land here (the backend is down or too slow).
+        # There is no 50x page, so answer directly instead of a 404.
         location = /50x.html {
-            root /app/html;
             internal;
+            default_type application/json;
+            return 502 '{"error":"The Termix server is not responding"}';
         }
     }
 }