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.
The spec wants a request that doesn't include any items to go 400 with validation errors. Without this, it'll 200 instead. I don't think this actually matters much, but this was required to keep the spec green.