Need lightweight self-contained HTML error pages

## Issue Description

We need lightweight self-contained HTML error pages that use the Creative Commons brand.
- should use Vocabulary
- should be simple and informative
- must be completely self-contained (all assets inlined)
- must be small (must not exceed 1.43MB)
- may provide instructions or next steps

For use on hosts:
- **403 Forbidden**
- **404 Not Found**
- **503 Service Unavailable**

For use on Cloudflare (potentially using custom error tokens):
- Challenges:
- Basic security
- Web Application Firewall
- **IP Firewall (Country block, Country challenge, IP (range) block)**
- Cloudflare errors:
- 502 Bad Gateway
- 504 Gateway Timeout
- **CF 52X**
- 10XX errors
- Always Online

The higher priority error messages are bolded.

## Related Links
- [Customizing Cloudflare error pages – Cloudflare Support](https://support.cloudflare.com/hc/en-us/articles/200172706-Customizing-Cloudflare-error-pages)
- [Troubleshooting Cloudflare 5XX errors – Cloudflare Support](https://support.cloudflare.com/hc/en-us/articles/115003011431-Troubleshooting-Cloudflare-5XX-errors)
- [List of HTTP status codes - Wikipedia](https://en.wikipedia.org/wiki/List_of_HTTP_status_codes)