Supplement QuoteExceededError with NotAllowedError for canMakePayments #485
Description
I think QuotaExceededError is the wrong type of error for canMakePayments, since the user agent can return it at its discretion - not just when a quota limit has been reached.
For example, a user agent may decide to not service a canMakePayments request when in a private browsing mode or similar, or may allow the user the option to decline to reveal their payment methods to the merchant.
In those cases a quota hasn't been reached, the call has simply been denied. I wonder if it would therefore be better to permit user agents to return NotAllowedError instead. It's somewhat of a semantic argument, but otherwise merchants could end up with a scenario when they call that API a single time and get a quota exceeded error, which doesn't make much sense.