changeset version should return a non-zero error code if there are no changesets #1815

In a repo with changesets configured but no changesets:

$ npx changeset version --snapshot canary
🦋  warn No unreleased changesets found, exiting.
$ echo $?
0

This makes it difficult to publish canary builds using changesets. For example the following CI job:

$ pnpm changeset version --snapshot canary
$ pnpm publish --tag canary

The expected behaviour is that we version our packages with a -canary-<something> suffix, and then publish packages with version x.y.z.canary-<something> to NPM

But in the case that there are no changesets, this will print the warning about no changesets, and then silently continue to publish our packages as x.y.z with the tag canary. This then causes issues if another CI job is intended to publish x.y.z with the tag latest, as that version is already published on npm.

I think the best solution would be for changeset version to return a non-zero exit code when there are no changesets.