packaging: engine-backup: Suggest --help on failure

Also added to --help a link to the wiki page.

Bug-Url: https://bugzilla.redhat.com/1064903
Change-Id: I6021095d229df8279f5b593f2a6619867d4fcc99
Signed-off-by: Yedidyah Bar David <didi@redhat.com>