Add error check to success test in README example

The Ruby SDK will return ```true``` for ```response.success?``` if the HTTP _request_ was successful (ie response is 200), however, the _transaction_ may not have succeeded due to PayPal business logic. The status of the transaction may be checked via ```response.payment_exec_status```.

See Paypal Docs at [1] for an explanation of these conditions.

[1] https://developer.paypal.com/docs/classic/api/adaptive-payments/ExecutePayment_API_Operation/#id092PA0EE05Z__id092PAL000UI