the default failure message is:
Your transaction could not be completed. Either your credit card information or billing address has problems and cannot be processed
Please return to checkout and try again.
You could add the response reason text to this if you like.
On the failure page, go to the bindings panel, click the plus button and select eCart -> checkout -> Local Checkout response. from the list of gateways, select Authorize.net AIM
this will add a local checkout binding group. You can add the response reason text to the failure page to give the reason for the failure.