refactor: attach error code to error message

Author: shenyih0ng

mypy/checker.py

@@ -2288,7 +2288,9 @@ def check_method_override_for_base_with_name(
                     and isinstance(defn, Decorator)
                 ):
                     self.fail(
-                        message_registry.CANNOT_OVERRIDE_CLASS_VAR.format(base.name), defn.func
+                        message_registry.CANNOT_OVERRIDE_CLASS_VAR.format(base.name),
+                        defn.func,
+                        code=codes.OVERRIDE,
                     )
 
                 if (