createiso: Skip if buildinstall fails
    
    If the ISO is meant to be bootable but lorax fails, there's no point in
    creating the ISO as it will not behave as expected.
    
    Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1574585
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>