Make UAs throw for invalid monetary amounts

[sideshowbarker]

No description provided.

[adrianba]

The problem with this change is that it doesn't deal with the updating case where throwing isn't a possibility. Just like in #249 it would have been easy to throw in the constructor in this situation but then the behaviour would be inconsistent in updateWith(). We didn't really want the bad update to cause the payment request to be terminated because doing so looks like a user agent bug and not a page bug. Also it will be easier to debug the problem when a user explains what happens.

So for the currency value I'd prefer a different proposal.

[adrianba]

Given that we already have the difference I outlined above for negative totals, I've updated the PR to apply the same difference to updateWith.

[sideshowbarker]

Thanks this looks great—will update the expected results in the corresponding wpt tests to match this.