First Fix

This fixes the following:

- Display PayPal Error message
- Fix duplicate payment detection
This can be reverted to it's prior state.  This was changed in 3.9.1 because things were not working.

https://github.com/wp-e-commerce/WP-e-Commerce/commit/f31872b169e336a01aa70cf53fdfa8dec25ebd8b#diff-42d4212bc6ad461f9ab90d2e10cf66daR29