Throw a decline exception instead of an invalid argument exception when the PO number / upload are empty

This will need to be backported to 8.x-1.x as well.

Here are the patches, one for each branch. I do wish we could actually surface the exception message in the UI, but ultimately I think we may just need some form enhancement to make it clear that the number would always be required and the file is optional. Not really sure of the intent behind the current form, to be honest.

Tagged a 1.4 release on the unsupported 8.x-1.x branch

I'll tag a full 2.0 release after marking this issue fixed.

Thanks for the collaboration @rszrama! My original client required a PO number. After this was contributed, a feature was contributed for the file upload of a copy of the PO. The instructions should be configured for the user in the gateway config so that the tasks are clear.

Happy to help!

Automatically closed - issue fixed for 2 weeks with no activity.