Handle orders that are already partially returned

This change allows for automatic processing even if an order is already
partially returned. If, for example, the payload indicates that we
should create a customer return for items a, b and c but item a was
already returned, this change allows for automatic processing of b and
c. Previously, this scenario would have resulted in a Wombat error.
got it, i think an error response does make sense, just not sure if we need to rely on a customer return building with no return items for that to be the case. this code is certainly an improvement though so don't let this hold it up.