FAILURE value is not a key-id

[sebbASF]

PR #180 prevented FAILURE messages from overwriting the key_id:

python-gnupg/gnupg.py

Line 337 in 0dab090

if not self.key_id: # don't corrupt the key id with 'gpg-exit nnnnn'

As far as I can tell, the value that is associated with the FAILURE key is not a key-id, so should never be used to set the key_id.
So whilst the PR fixed the immediate issue, the key_id could still end up with an invalid value.

I don't know whether the UNEXPECTED message value can be a key_id.

[vsajip]

Closed in #187.