Did as you suggest, but…
Did as you suggest ( I think) which inserted the following at line 12 of checkout_failure.php:
and also made some changes to AuthNet_PHP.php.
Uploaded both and submitting on the checkout page still returns me to cart.php with the single phrase on the page - "The cart is empty"
PS Clicking the plus button adds eCart->Local Checkout Response to the page but I'm afraid I don't understand - "Then you can drag and drop that value on the page so that you can see what the error is" as that value is already there.