docs(design): Add general content guidelines for error messages

[chris-at-jobber]

Motivations

Designers often seek general guidance on error messaging. While we have component-specific guides, we don't have them generalized.

Changes

Added

• Guidance on general error messaging to Voice and Tone

Testing

View docs locally or in the Cloudflare build

---

In Atlantis we use Github's built in pull request reviews.