using the payment module function get_error

[torvista]

While investigating this, I see it is used only by /pages/checkout_payment/header_php.php,
for example
In the header:

if (isset($_GET['payment_error']) && is_object(${$_GET['payment_error']}) && ($error = ${$_GET['payment_error']}->get_error())) {  $messageStack->add('checkout_payment', $error['error'], 'error');} 

In the payment module:

  function get_error() {
 $error = array(
'title' => MODULE_PAYMENT_AUTHORIZENET_AIM_TEXT_ERROR, 
'error' => stripslashes(urldecode($_GET['error'])));
return $error;
} 

I am assuming this is a way to show a relevant error message to the user when an (offsite?) payment fails and they are redirected back to the checkout_payment page by the gateway.

I cannot get this to work, I assume that the url should have something like:
?payment_error=the_module_name&error=message
but this just goes to page not found.

I find no mention of this anywhere so can someone please explain how to use it.
thanks
Steve