Reject acceptedPromise if payment method identifer is invalid

[adrianhopebailie]

Per suggestion here for consistency with fail fast design: #536 (comment)

The payment identifiers provided in a request should be valid (per the PMI spec) and can either be checked as part of the show() algorithm at step 11 here or earlier at step 4.2 of the Payment Request constructor here although this is likely more processing than desired inside a constructor.

Assuming this check is implemented in the show() algorithm it should result in the promise being rejected. The error to use is up for debate...

URL parsing error seems best for a URL identifier unless the issue is specific to payment method identifiers (not using https etc)

[marcoscaceres]

Closing as duplicate of #464.

@adrianhopebailie, I'm about to send a PR that ignores the invalid ones during construction as per #464.

Checking what UAs do in the tests (coming also with PR).