MSB Processing Confirmation Handler

Identify the circumstances under which a MSB API cart process request may fail, and handle it properly. Determine whether this means automatically refunding the order, ignoring it, resetting a user's cart, etc.
Currently, this simply writes an error to the user's failure page:
https://github.com/apc-gmhs/mustang-mug/blob/a1b528077296871dc19ff18318f5a6278389bf5c/src/routes/cart/process.js#L60-L67