DPS/Payment Express: Unsuccessful transactions being mis-handled as successful

There is an issue in the code for the DPS/Payment Express payment module which prevents handling of declined transactions.

With this issue, a contributor or even registrant could be redirected to DPS for payment. The payment would be declined, but, upon return to the website, the contributor or registrant would be presented with the "thank you" page and the transaction would be marked as "completed".

The attached file corrects this issue so that declined transactions are not marked as complete and the contributor is returned to the main contribution or event page to re-start the process.