Critical regression from 964fb14

This breaks onCaptured, since it checks for `TotalOutstanding` via `Payable->TotalPaid()`, which in turn uses `the Order->Payments()` relationship. Since `Status=Captured` isn't written at this point, the result is `>0`, failing the order.
Merge pull request #57 from silverstripe-iterators/pulls/void-on-failure-fix

Void on failure fix