Check if tag already exists before publishing

[ianstormtaylor]

If the Git tag for the version already exists, np will fail, but only after bumping the version in the package.json and pushing a new commit to master.

[sindresorhus]

Yes! That has happened to me multiple times before. Should really be fixed in npm version, but I doubt anything will happen there, so we can fix it here. What should happen if the tag already exists? I assume you want it to fail in the Prerequisite check?

[ianstormtaylor]

Yup, I think failing as early as possible if it already exists sounds good!

[SamVerschueren]

Definitely a good idea!

[sindresorhus]

I opened an npm issue for this so it can be fixed there eventually: npm/npm#13660