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.
build script: Mass fix trap ... ERR INT TERM -> trap ... ERR.