[Warehouse] Prevent negative stock.

Task: https://accounts.openerp.com/web#id=10130&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.KCBJWsVnxINg746v
@bst-odoo , i have improved string into "cannot" as per your comments.
@jco-odoo ,i have improved if condition into move.location_id.usage=='internal' as per your comments.
Thanks.