server {
        listen 80 default_server;
        listen [::]:80 default_server ipv6only=on;

        . . .

        error_page 404 /custom_404.html;
        location = /custom_404.html {
                root /usr/share/nginx/html;
                internal;
        }

        error_page 500 502 503 504 /custom_50x.html;
        location = /custom_50x.html {
                root /usr/share/nginx/html;
                internal;
        }

        location /testing {
                fastcgi_pass unix:/does/not/exist;
        }
}