refs https://github.com/TryGhost/Toolbox/issues/410
- The 'private' value in 'Cache-Control' response header for all errors made it impossible for shared caches (e.g.: Fastly, Cloudflare) to cache 404 responses efficiently.
- The change substitutes 'max-age=0' which should not effect the browser cache behavior but would allow shared caches to process such requests efficiently.
- A more loose caching logic only applies to 404 responses from GET requests that are not user-specific (non-authenticated, non-cookie containing requests)
7b009bf1fe