querier: Improve error message when ERQLExpression fails on some entity (closes #2267081)
When an RQL variable is indeed "constant" (we know it's eid) ERQL expression are
not inserted but checked prior query execution. If the session have no read
access to the entity an `Unauthorized` exception is raised.
This changeset add a meaningfull message to this exception::
No read acces on "X" with eid 42.
