core: backup.sh return code always 0 even on error

Removing the usage call in case of error since the usage method exists
with 0 value

Change-Id: Iff32ce8ca0ab8763ee5bbe93a977397df548d4ea
Signed-off-by: Eli Mesika <emesika@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=966980