[clank][spc] Add an AbortError response for merchants when users cancel

Currently, when users cancel the SPC dialog, a `NotAllowedError` is
always returned. Instead of always returning this, add an additional
`AbortError` that is returned when the user wishes to cancel and NOT
continue with the payment. This is different from `NotAllowedError`
which indicates that the user still wants to continue with the payment
but without SPC.

Not that this is hidden behind the SPC Fallback feature flag.

Implementation: http://screen/iws8Ma9ZoRvLFV7
Proposal: https://github.com/w3c/secure-payment-confirmation/issues/275

Bug: 408026839
Change-Id: I112358f7ab844c5614749c3927144f736731fba2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6488741
Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Commit-Queue: Darwin Yang <darwinyang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452847}