Improve error handling for purchase batch

so error will display in browser when applicable

tailbone/views/purchasing/ordering.py

@@ -428,8 +428,11 @@ class OrderingBatchView(PurchasingBatchView):
                 self.handler.add_row(batch, row)
 
             # update row quantities
+            try:
                 self.handler.update_row_quantity(row, cases_ordered=cases_ordered,
                                                  units_ordered=units_ordered)
+            except Exception as error:
+                return {'error': six.text_type(error)}
 
         else: # empty order quantities