Add watchdog call when payment fails

It's useful to know why payments are failing. Patch introduces wachdog call with details on response.

Also a small tweak to the $return value. Put an extra, empty element on the beginning of the array. That way the order of the response codes correlates with that of the authorize.net documentation, http://www.authorize.net/support/AIM_guide.pdf, which begins at 1 rather than 0.