How to handle failed authorization

I'm implementing a payment gateway module which works fine as long the authorization is successful.

To route the callback from the credit card company I've set-up a success and a failure link as described in http://drupal.org/node/1040318.

url('checkout/' . $order->order_id . '/payment/return/' . $order->data['payment_redirect_key'], array('absolute' => TRUE))
url('checkout/' . $order->order_id . '/payment/back/' . $order->data['payment_redirect_key'], array('absolute' => TRUE))

If an authorization is accepted it redirects to

CALLBACK_commerce_payment_method_redirect_form_validate
CALLBACK_commerce_payment_method_redirect_form_submit 

There I create a new transaction, store it and the checkout process is finished by commerce.

However if the authorization fails, non of the callbacks described in commerce_payment.api.php is called. The user is silently redirected back to the checkout page.

How can I do custom processing when the failure back link is called from the credit card company? I want to show the error message from the credit card company to the user.

For now I found this solution. But I don't think that it's really the intention doing it that way:

I implemented hook_commerce_checkout_router to get a hand on the data sent from the credit card company, store it in the $_SESSION variable and check in CALLBACK_commerce_payment_method_submit_form whether there's something stored in $_SESSION.

This is my code:

function commerce_datatrans_commerce_checkout_router($order, $checkout_page) {
	// check whether this is a failed payment atempt
	$data = $_POST;
	if (isset($data) && isset($data['status']) && $data['status']=='error' && arg(3)=='back' && arg(4)==$order->data['payment_redirect_key']) {
		$_SESSION['commerce_datatrans_error'] = $data;		
		//_commerce_datatrans_transaction_save($payment_method, $order, $data, COMMERCE_PAYMENT_STATUS_FAILURE);
	}
}

function commerce_datatrans_submit_form($payment_method, $pane_values, $checkout_pane, $order) {
	
	$data = isset($_SESSION['commerce_datatrans_error']) ? $_SESSION['commerce_datatrans_error'] : NULL ;
	if (isset($data)) {
		drupal_set_message(_commerce_datatrans_map_error_code($data['errorCode']), 'error');
		unset($_SESSION['commerce_datatrans_error']);
	}
	
	return array();
}

How should this be implemented in a clean way?