Allow web-based payment handlers to report back internal error via Payment Request API

[stephenmcgruer]

See w3c/payment-request#1050 and w3c/payment-request#1040, where the Payment Request API is now defining more explicit steps to let payment handlers report back internal errors (as differentiated from user abort).

The Web-Based Payment Handler spec will need to be updated to match, likely by being specific about what happens when the promise passed to respondWith is rejected (and with what types, to let them choose between AbortError and OperationError). It will also have to consider backwards compatibility.