Do not raise ValueError when searching for an ERP5 user which do not exists or due to security settings is not accessible to logged in user. Instead return None and let application logic choose appropriate action.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@25310 20353a03-c40f-0410-a6d1-a30d3c3de9de