Add support for PaymentValidationErrors

[ianbjacobs]

With the addition of retry() to Payment Request, we will want payment handlers to have access to PaymentValidationErrors.paymentMethod.

[ianbjacobs]

@rsolomakhin and I discussed some scenarios, including:

• User billing address is for some reason incorrect, and payment handler should be aware of that.
• Specific payment instrument managed by the payment handler cannot be used, and the payment handler needs to know which one.

[ianbjacobs]

Adding a note here from a 2018 discussion:

• Do we need a PaymentRetryEvent event for payment handlers that has the original paymentRequestEventInit data + previous response + error data.