Handle customer returning from a failed transaction

As a merchant, I want to let customers know if and why their transaction failed, and let them try again, so that they can try again to complete their transaction successfully

Direct the user back to the credit card form
Show human readable error message based on the Cybersource response codes (the text messages that come with these codes are technical and not suitable for customers). See here: https://forum.foxycart.com/discussion/311/customizing-payment-gateway-reason-code-messages-in-foxycart/p1 (note these are used in the Penn app)
Verify the Cybersource return signature (same as in the story for a successful transaction)

tweag / cybersourcery_demo_site

Handle customer returning from a failed transaction #8