Offer custom 404.html / 50x.html pages

[m4dz]

Error pages, like 404 or 50x pages, should be served directly by the frontend web server (nginx, caddy, apache), so Cozy apps have just to send 404/50x status code that can be catched.

Proxy is always installed in Cozy plateforms, I so suggest that it embed in its assets those error pages (with inlined styles / scripts if needed). We must also provide configuration exemples for aforemetioned mainstream webservers , that points to those pages in the FS, to catch those status errors.