Better demoing error message

[michaeljguarino]

Summary

Apparently users don't fully grok what to do here, perhaps this makes it more clear.

Test Plan

Checklist

• [x] If required, I have updated the Plural documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] I have added a meaningful title and summary to convey the impact of this PR to a user.
• [x] I have added relevant labels to this PR to help with categorization for release notes.