Re: [w3c/payment-request] retry and fine grained error recovery for fields (#647)

Seeking input:

 * @domenic, feedback on IDL and general design would be appreciated.  
 * @aestes, are there cases that are not covered (i.e., that you couldn't then map to `ApplePayError`)? 
 * @mnoorenberghe, @edenchuang, implementable? 

## Shipping address errors
Looking back over the discussion, I think having a `PaymentError` interface might be confusing and a bit heavyweight: Confusing, in that it's not an `Error` in the JS/DOM sense (has no stack, for example). Heavyweight, in that it simply models a triple, but provides no other utilities.

As an alternative, I think we can just use dictionaries to achieve the same result:

```JS
dictionary AddressErrors {
  DOMString addressLine;
  DOMString city;
  DOMString country;
  DOMString dependentLocality;
  DOMString languageCode;
  DOMString organization;
  DOMString phone;
  DOMString postalCode;
  DOMString recipient;
  DOMString region;
  DOMString regionCode;
  DOMString sortingCode;
};

partial dictionary PaymentDetailsUpdate {
  AddressErrors shippingAddressErrors;
}
```

Note: `AddressErrors` could be reused for billing address errors, once the Payment Request API actually supports billing addresses properly (https://github.com/w3c/payment-request/issues/27 and https://github.com/w3c/payment-request/issues/550). 

Note: Errors for redacted fields could be ignored by the UA (e.g., complaining about shipping address' `phone` during the "payment request" phase would be ignored, but would be honored when retrying the payment response).   

### Usage example

```JS
pr.onshippingaddresschange = ev => {
  const shippingAddressErrors = {
    "city": "FarmVille is not a real place.",
    "postalCode": "Doesn't match a known zip code for your country.",
  };
  event.updateWith({ shippingAddressErrors });
}
```

## Payer-related errors
The next class of errors deals specifically with payer related errors (name, email, phone):

```JS
dictionary PayerErrors {
  DOMString email;
  DOMString name;
  DOMString phone;
}
partial dictionary PaymentDetailsUpdate {
  PayerErrors payerErrors;
}
```

### Example

```JS
const payerErrors = {
  email: "The domain name is invalid",
  phone: "Invalid format",
};
// We don't have a means to use this object yet... that will be `retry()`.
```

Thoughts, before I spec it up? 

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