[FIX] warning: make blocking warning actually doing something (opw 607380)

Instead of raising an exception (which only interupt the call of the onchange methods), return a dictionnary to set the forbidden partner or product to an empty value.
The call is not blocking in a way to forbid creating a record that uses this partner/product but it is not possible to select it through the web interface.

bzr revid: mat@openerp.com-20140507113940-d8iynfl1lcskht63