git tag -s: TAG_EDITMSG should not be deleted upon failures

Before I've now set my default signing key id in my ~/.gitconfig, I've run at least half a dozen times into the case where I'm running "git tag -s $tagname", carefully preparing a tag message, saving the file & exiting from the editor, only to be greeted with an error message that no key could be found for my (deliberately host-specific) email address, and my message gone. If it would keep the TAG_EDITMSG file (like git commit seems to be doing with COMMIT_EDITMSG anyway), I could rescue the message from there. I relentlessly assume that this small change would also make a handful of other people happier.

Christian.