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

[Christian Jaeger <christian@jaeger.mine.nu>]

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.