Fix error message in type analyzer

Author: brianschubert

mypy/typeanal.py

@@ -1869,7 +1869,8 @@ def bind_function_type_variables(
         defs = []
         for name, tvar in typevars:
             if not self.tvar_scope.allow_binding(tvar.fullname):
-                self.fail(message_registry.TYPE_VAR_REDECLARED_IN_NESTED_CLASS.format(name), defn)
+                err_msg = message_registry.TYPE_VAR_REDECLARED_IN_NESTED_CLASS.format(name)
+                self.fail(err_msg.value, defn, code=err_msg.code)
             binding = self.tvar_scope.bind_new(name, tvar)
             defs.append(binding)