workaround for
        apt: Provide meaningful exit codes for gpg failures
        https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745735
    W: A error occurred during the signature verification.
    To catch situations such as:
        The repository is not updated and the previous index files will be used.
        GPG error: http://deb.torproject.org stable Release: The following signatures were invalid: KEYEXPIRED 1409325681 KEYEXPIRED 1409325681 KEYEXPIRED 1409325681 KEYEXPIRED 1409325681
    That apt-repository would otherwise be silently ignored without error notification.