provide better error message when iSCSI luns list is empty

Change-Id: I3395502d1af4e16c21db5d8dec34137a3b1f82bb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1133937
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>