Also change the behaviour a bit, don't raise an Exception directly, constraints
should *not* raise exceptions, simply use _generateError. What we should do is
add support for different severity in Constraints framework.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18369 20353a03-c40f-0410-a6d1-a30d3c3de9de