[IMP] stock: display translated selection value in message

When a user tried to delete a done or canceled picking, the error messages used to display the key of the selection field ('done' or 'cancel') which was surprising in other languages than English. This patch takes the string value of the selection field, keeping the context to get the translated value (opw 613068)
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32