Deferred intent - Display a detailed error message to customers on card declines

[james-allan]

Fixes #2920

Changes proposed in this Pull Request:

This pull request makes sure we display a more detailed and specific error message when a card declines.

Before	After

Testing instructions

1. Enable UPE checkout.
2. Add a product to your cart
3. On checkout use one of the failing payment methods:
   • 4000000000009995 - Insufficient funds decline
   • 4000000000000069 - Expired card decline
   • 4000000000000127 - Incorrect CVC decline
4. On add/deferred-intent you will see a generic error message not indicative of the error that occurred. See before screenshot above.
5. On this branch a more specific error will be displayed.

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)

[mattallan]

• Classic checkout 👍
• Block checkout 👍

Tested different failing cards

• 4100000000000019 - high risk (fraud)
• 4000000000000002 - generic decline
• 4000000000009995 - insufficient funds
• 4000000000000069 - card has expired

Nice fix @james-allan !