Braintree: Handle 'Processor Declined' errors

[lukesutton]

Currently this error state generates a result where #successful? returns false as expected, but which has no errors against it. This is because the Braintree library doesn't capture this particular error in it's errors collection; which is really really annoying.

An alternate way of handling it would be to explicitly check for the error and coerce it into something useful.

[jordizle]

See this link. It describes accessing the 'Processor Declined' in more detail:

https://developers.braintreepayments.com/javascript+php/reference/response/transaction#processor-declined