Expose a failure reason in `PaymentFailure`

[TheBlueMatt]

When a payment fails completely, we should at least tell the user if its because the recipient rejected it or if we ran out of attempts.

[alecchendev]

I can pick this up! Would this mainly be a simple enum added to Event::PaymentFailed with some doc comments? In which case I can try and open a PR tomorrow

[TheBlueMatt]

I think so yea. I believe one of the cash app developers was already gonna try to pick this up this week.

[alecchendev]

Oh okay all good then 👍

[rloomba]

hey @alecchendev! feel free to take this ticket

[alecchendev]

Sounds good @rloomba, thanks!