gform_after_submission return an error back to the form

I am making an HTTP api request to a web service in the gform_after_submission hook... If the web services returns an error how would I return an error back to the validation from this hook?

It seems like there is not a way to return an error at the gform_pre_submission hook either?
Anyone?

thanks... I tried this
$form["is_valid"] = false;
return $form;

But that did not seem to help...