We have had lots of feedback that merchants need to explain errors to
customers during the payment request. For example, if a shipping address
is not acceptable then the site must be able to indicate why.
This change adds an `error` string to the `PaymentDetails`, which may
only be used as part of `updateWith`.
- https://github.com/w3c/browser-payment-api/pull/257
Comment 1 by hs1217....@samsung.com
, Sep 19 2016