Rewrite the code of forcing authorization, so that Unauthorized is raised, only if authorization is forced in a given context, and a document can be retrieved in an unrestricted way. Otherwise, a traversal fails for any attribute that does not exist with Unauthorized, and indexing may fail.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@21255 20353a03-c40f-0410-a6d1-a30d3c3de9de