[w3c/payment-request] Add generic `error` member for `.retry()` (#729)

when `retry()`ing, it would be nice to just have a generic error string. For example:

```JS
response.retry({error: "Oops, our server timed out. Please try paying again."})
``` 


-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/payment-request/issues/729