Change: Updates the output from an Error to Stderr when listing variables where none are set

[CGoodwin90]

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated.
• Changelog entry has been written

When running list variables on a project/environment where none exist an error is returned alongside the cli Usage text. Rather than returning an error, this updates the cmd to output the message via Stderr with an empty table (similar functionality to how the api currently operates) and doesn't unnecessarily display the Usage text. Also updates the output package to handle the error strings.

Closes #296

[shreddedbacon]

Sweet, minor quality of life improvement. Might be worth checking other things that return errors and handling them in a similar fashion