For example, if we get the `expired_card` error code back from Stripe,
we should show a specific error to the user.
This introduces some TODOs around error messages in a couple of spots.
We will revisit this when implementing PayPal monthly donations, as is
explained in the comments.
I tested this by using some of [Stripe's test numbers][0] (as well as
automated tests).
[0]: https://stripe.com/docs/testing#declined-payments
8c7c35b887