This gist shows how to setup a custom error_page on nginx when using a simple upstream server

nginx.conf

server {
	location / {
		proxy_pass http://upstreamserver;
		# or
		# try_files $uri $uri/ /index.html;
	}
	
	error_page 500 502 503 504 /50x.html;
	location = /50x.html {
		root /var/www/error-pages/;
	}
}