Return a 403 instead of 200 when trying to delete a user with posts

See [this commit][1] for more info

[1]: https://github.com/discourse/discourse/commit/bd352a17bff9019db21196d70cd9829e678a33be

app/controllers/admin/users_controller.rb

@@ -382,7 +382,7 @@ class Admin::UsersController < Admin::AdminController
         render json: {
           deleted: false,
           message: "User #{user.username} has #{user.post_count} posts, so they can't be deleted."
-        }
+        }, status: 403
       end
     end
   end