Update Tor's GPG key, check return codes, set -e
The old key expired and we have a new one (https://gitweb.torproject.org/debian/torproject-keyring.git/blob/HEAD:/keyrings/deb.torproject.org-keyring.gpg). Update ec2-prep.sh with the new key and build new images. Also update the script to set -e and check return codes.