SLING-3296 : more explicit exception messages, part 2

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1573739 13f79535-47bb-0310-9956-ffa450edef68
SLING-2945 : checking early for resource==null

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1573742 13f79535-47bb-0310-9956-ffa450edef68