warn on expired gpg signing keys instead of just failing

Currently when a signing key expires, smartgit only shows the attached message.
The log file also only outputs that gpg2 failed to sign a commit.

I'm not sure if you actually get more output by git/gpg if it fails to sign a commit with an expired key. If so showing that info or logging it at least would be nice.
In either way it would also be nice if smartgit does warn that this might be the reason, or could just make a quick check in the settings dialog if the specified key is still valid.

I know that it's a little bit of extra feature, but this really threw me off when it just stopped working after some years and the "user experience" is really bad. I just happened to invoke gpg2 -k and finding that the entry for my key is actually expired. Just re-running the smartgit command displayed below to debug it by yourself is obviously not possible as the tmp file is gone.