Validate packing at shipment creation
For now, the carrier validation is only done on the packed transition. But often it is too late to fix the problem. For example, UPS can require the phone number of the customer but we do not have access to the customer at that time.
I think the validation should start to be done at the creation like that when the sale is processed and the shipment created, the error is raised. There are still the issue of the shipment when the invoice is paid, but I guess in this case the information could be collected. And any way, it is not the warehouse employee that can fix it.