Validate shipment before create shipping

This ensure the shipment is still valid to create shipping for the carrier.
Otherwise it raises a nice error message.

issue9072
review303421002

stock.py

@@ -204,4 +204,9 @@ class CreateShipping(Wizard):
     start = StateTransition()
 
     def transition_start(self):
+        pool = Pool()
+        ShipmentOut = pool.get('stock.shipment.out')
+        shipment = ShipmentOut(Transaction().context['active_id'])
+        method_name = 'validate_packing_%' % shipment.carrier.shipping_service
+        getattr(shipment, method_name)()
         return 'end'