Actually return a 404 from the not found page

uitwww/__init__.py

@@ -35,7 +35,7 @@ def create_app(data_path):
 
     @app.errorhandler(404)
     def not_found(error):
-        return flask.render_template("404.html")
+        return flask.render_template("404.html"), 404
 
     return app