fix namespace, add error checking
    
    The containers namespace in distgit and pkgdb have changed from
    docker to container in order to highlight that we are targeting all
    OCI compliant container runtimes and not just one specific one. This
    patch handles the migration.
    
    This patch also adds further error checking to flr-koji for
    container rebuilds in order to gracefully handle various failure
    scenarios.
    
    Signed-off-by: Adam Miller <maxamillion@fedoraproject.org>