Work better with piping

When output is piped, e.g.

```
octoherd run myscript.js | my-other-cli-app
```

Then we should write all logs to stdout, and all other things to stderr.

> In short: stdout is for output, stderr is for messaging.

—https://medium.com/@jdxcode/12-factor-cli-apps-dd3c227a0e46

I'm still learning about CLI best practices. Besides the article linked above, https://clig.dev is fantastic