[w3c/payment-request] PaymentDetailsUpdate needs to include payerErrors (#766)

There is bug in the current spec, whereby merchant can't signal payer-related fixes via `updateWith()`:

```JS
// user tries to fix email
response.onpayerdetailschange = ev => {
   if (!isValidPayerDetails(response)) {
       const payerErrors = getPayerErrors(response);
       // Oh wooops, `payerErrors` doesn't exist on `PaymentDetailsUpdate`
       ev.updateWith({ payerErrors });
   }
}
```

Same applies to `paymentMethod` errors.  

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https://github.com/w3c/payment-request/issues/766