Impossible to upload a Purchase order file only

The attached patch adapts the error message if the file field isn't present on the form due to the private path not being set.

This is a bug. The feature was contributed and I missed this in the review. Further, manual testing shows that the Purchase order output is not useful or accurate when a file is used. Fixing that too.

The core of this issue is that the logic about what information is needed in PurchaseOrderGateway::createPaymentMethod is not correct.

@jsacksick Please review and comment on the MR above.

Omitting the validation function so I can address 📌 Rename the payment method add form class Active