configure nginx error page for reverse proxied services

The basic idea is to use nginx for

• turn on proxy_intercept_errors on; in the downstream service configuration.
• define the error page at the upper level server configuration.

Refer to this article:

• https://erikzaadi.com/2014/06/23/handle-proxy-404-in-nginx/
• https://guides.wp-bullet.com/nginx-redirect-404-errors-to-homepage-wordpress/