When gpg failed while decrypting, encrypting or looking up a key
fingerprint, the error gave only gpg's exit status, so the user could
not tell why. cmd.Output keeps gpg's stderr in the *exec.ExitError it
returns; the three errors now end with that stderr, trimmed. When gpg
wrote nothing to stderr, the error is left as it was.
gpg's stderr can run to more than one line, and the error then does too.
The regression test gives gpg data that is not an OpenPGP message
instead of one encrypted to a missing key: gpg reports both the same
way, and this case starts no gpg-agent the test would have to stop.
Model: opus-5-5
sneak