Update error handling

Now fails if the payment exec status is not successful status:
CREATED
COMPLETED
INCOMPLETE
PROCESSING
PENDING

Add test
Merge pull request #9 from OpenBuildings/fix-error-handling

Previously we've only checked the acknowledgement field to determine if the operation is successful.

[According to the `ExecutePayment` API reference](https://developer.paypal.com/webapps/developer/docs/classic/api/adaptive-payments/ExecutePayment_API_Operation/#id092PA0EE05Z__id092PAL000UI) we must check the `paymentExecStatus` as well.