I have installed Squid 3.5.12 on Ubuntu 16.04.4 LTS x64, everything is working fine with blocking everything and allowing a whitelist-only, except for when trying to access a website not on the whitelist, if it's HTTP, I get:
ERROR The requested URL could not be retrieved
and if it's HTTPS, I get:
This site can’t be reached
How to get them both display the same message or if possible a custom HTML page?