conncheck: fix progression on failure
    
    traceback.format_exc() does not take exception object as an argument.
    This made Python 3 get stuck amid ipa-replica-conncheck, probably
    because it was waiting for a thread to finish.
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Felipe Volpone <fbarreto@redhat.com>