Michael Tolan

You're correct about bad input entering the process upstream, I'd been following some guidance pulled from elsewhere to use the short git revision as a tag. sh "git rev-parse --short=12...

That seems reasonable, but it's also possible for the user to specify a tag in the ```tag``` and ```push``` methods, so I suspect we'd want the same error handling there....

Apologies that this is pretty light on expected behavior and architecture. My golang knowledge is still a bit rudimentary, and I wouldn't presume to suggest where the most central changes...