tools: check tag is on github before release

PR-URL: https://github.com/nodejs/node/pull/9142


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>