[w3c/browser-payment-api] Unambiguously specify UA requirements for instances that are not valid (#235)

The [PaymentCurrencyAmount section of the spec](https://w3c.github.io/browser-payment-api/#paymentcurrencyamount) states MUST requirements for the `value` field of `PaymentCurrencyAmount` instances, but the spec doesn’t explicitly define what the requirements are if a `value` field in an `PaymentCurrencyAmount`  instance is found to not be valid.

Specifically the spec doesn’t state, e.g., that a conforming UA must throw if a `value` field in an `PaymentCurrencyAmount` instance is found to not be valid.

>From the current language of the spec, it’s not even clear if the “must“ requirements stated for the `value` field are requirements that UAs must (somehow) enforce or whether they are simply requirements that authors must follow (that is, document-conformance requirements).

Another case where the spec similarly states MUST requirements but does not explicitly state what if anything must happen for cases of non-conformance with those requirements is in [the spec text regarding the uniqueness of the `id` field in a `PaymentShippingOption` instance](https://w3c.github.io/browser-payment-api/#paymentshippingoption-dictionary)

Maybe the fix for those cases is, just always state the specific requirements in a “the _foo_ constructor must act as follows“ list, in the way that, e.g., [the UA requirements for the `total` field are stated](https://w3c.github.io/browser-payment-api/#paymentrequest-constructor):

> If the first character of `details.total.amount.value` is U+002D HYPHEN-MINUS, then throw a _TypeError_

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The context for this issue is that I’m working on evaluating the [tests currently in the `paymentrequest-constructor` branch of the web-platform-tests repo](https://github.com/w3c/web-platform-tests/blob/paymentrequest-constructor/payment-request/paymentrequest-constructor.html), which can at the moment be run in a browser from [w3c-test.org/submissions/3441](https://w3c-test.org/submissions/3441/payment-request/paymentrequest-constructor.html) (and which are basically just copies from Chromium [third_party/WebKit/LayoutTests/payments/payment-request-interface.html](https://chromium.googlesource.com/chromium/src/+/master/third_party/WebKit/LayoutTests/payments/payment-request-interface.html).)

For those tests:

1. The tests check to make sure the UA throws if the `currency` field is not valid. (Those tests seem to me to clearly be incorrect tests, since [for `currency` the spec unambiguously states, “any string is considered valid”](https://w3c.github.io/browser-payment-api/#dfn-currency).)

2. The tests check to make sure the UA throws if the `value` field does not conform to [the MUST requirements the spec states for the `value` field](https://w3c.github.io/browser-payment-api/#dfn-value).

However, for (2) above, because the spec never explicitly states the UA must throw if the `value` field doesn’t conform to the specified requirements (in fact it doesn’t clearly state anything that must happen), it’s not clear to me that a UA must throw (as the test expectations are checking for).

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