[w3c/payment-request] teach retry() about payerErrors (#721)

BUILD ON #720 - This is part 2. It adds:

 * `retry()` argument for validation errors: `retry(PaymentValidationErrors errorFields)`
 * `PaymentValidationErrors` dictionary
 * `PayerErrorFields` dictionary 

The following tasks have been completed:

 * [x] Confirmed there are no ReSpec errors/warnings.
 * [ ] Added Web platform tests (link)
 * [ ] added MDN Docs (link)

Implementation commitment:

 * [ ] Safari (link to issue)
 * [ ] Chrome (link to issue)
 * [ ] Firefox (link to issue)
 * [ ] Edge (public signal)

Impact on Payment Handler spec? 
Unknown.
 
## Example
Via payerErrors, the user now knows what's actually wrong with the payment.  

```JS
async function doPaymentRequest() {
  const request = new PaymentRequest(methodData, details, options);
  const response = await request.show();
  try {
    await recursiveValidate(request, response);
  } catch (err) { // retry aborted.
    console.error(err);
    return;
  }
  await response.complete("success");
}

async function recursiveValidate(request, response) {
  const promisesToFixThings = [];
  const payerErrors = await validatePayerInput(response);
  if (!payerErrors) {
    return;
  }
  await response.retry({ payerErrors });
  return recursiveValidate(request, response);
}

doPaymentRequest();
```

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-request/pull/721

-- Commit Summary --

  * WIP: retry() shell
  * define aborting and retrying
  * return promises rejected
  * complete() throws if retrying
  * Link to initial tests
  * Guard against calling request.abort() again
  * fix typo
  * Refactor pasta
  * Make it clear the UI is shutting down
  * remove redundant checks
  * Clarify closing, add asserts back in
  * s/request/reponse typo
  * Remove redudant check
  * Refactored .retry()
  * Teach retry() about payerErrors

-- File Changes --

    M index.html (293)

-- Patch Links --

https://github.com/w3c/payment-request/pull/721.patch
https://github.com/w3c/payment-request/pull/721.diff

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