How can a Browser-based Payment App cancel the payment flow?

[ianbjacobs]

Migrated from
w3c/webpayments#128

[adamroach]

The current service worker proposal addresses this with:

The user agent receives a failure response from the payment app through rejection of the Promise. The user agent MUST use the rejection reason to reject the Promise that was created by PaymentRequest.show().

I think that should d be sufficient for canceling the payment flow, although it's possible I've overlooked something.

[rsolomakhin]

Can we let the user select a different app, if one of the apps fails?

[ianbjacobs]

At the 23 November 2016 task force call, there was support for not prohibiting a "retry with other matching payment app" behavior by the user agent.

http://www.w3.org/2016/11/23-apps-minutes.html

[tommythorsen]

I've added a PR that addresses cancellation here: #72