Work around GGO gnome-software/582 till it's fixed
    
    We're getting a spurious "Authentication required" for a
    background refresh of the system repos sometimes, depending on
    whether it happens while we're at a tty. This is now accepted
    by upstream, so let's handle it and make it a soft failure for
    now.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>