A boolean flag to serve error pages using a new GET sub request.
Defaults to false, which means it serves error pages using file
502 => /home/www/htdocs/errors/maint.html;
404 => /static/404.html, 403 => /static/403.html, subrequest => 1;
This configuration serves 502 error pages from file system directly
assuming thats when you probably maintain database etc. but serves
404 and 403 pages using a sub request so your application can do some
logic there like logging or doing suggestions.
When using a subrequest, the subrequest should return a regular 200 response.