LSNBLDR-279; show warning message if no items visible on page


git-svn-id: https://source.sakaiproject.org/svn/lessonbuilder/trunk@128383 66ffb92e-73f9-0310-93c1-f5514f145a0a
LSNBLDR-279; clarify text of warning messaage


git-svn-id: https://source.sakaiproject.org/svn/lessonbuilder/trunk@128384 66ffb92e-73f9-0310-93c1-f5514f145a0a