gather: Track multilib that doesn't exist
    
    If a multilib package fails to be added (broken dependencies, or it
    doesn't exist at all), we want to track it and eventually stop the
    iterations.
    
    Merges: https://pagure.io/pungi/pull-request/1071
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>