fix api decorator on _popup_exceptions

This method returns an action dictionary, and with api.one a list was
returned. For that reason no window was displayed. Fixes #59
fix: create() returns a model instance in v8
refactor _popup_exceptions a bit
refactor Class -> class_model
Merge pull request #60 from lepistone/fix-display-exception

Fix display exception
fix future exception for dropshipping

In a previous commit by yours truly, I disabled the stock-based checks
in dropshipping situations, as was suggested in #47.

The idea was correct, but the method future_orders_are_affected needs to
return False to skip the check (it means that future orders are _not_
affected, and we are cool).