r2242 removed the case where we wouldn't be abdle to access the last
version of an object with loadBefore, and its test code is reused here to
check the "not visible" case.
r2250 modified the response in case where an internal error occured.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2254 71dcc9de-d417-0410-9af5-da40c76e7ee4