I thought I'd provide an update on my attempts at having a custom error
page returned at an OPAC.

I started looking into hacking the error pages in Firefox itself (thanks,
Kaleb!), but due to code-signing requirements, it seemed like a rabbit hole
I'd want to avoid, at least for now (even though Easter is upon us :-).  So
I set up a Squid proxy server with the necessary network restrictions and a
custom error page as previously reported, and I thought I nailed it until I
found that this only works for HTTP requests.  If it's HTTPS, the browser
returns its own error.  This behavior is apparently ubiquitous with all
browsers according to what I found.  Here's the most exhaustive posting
I've found on the subject:
https://squid-users.squid-cache.narkive.com/527KMD5K/squid-custom-error-page

I don't suppose there's an extension that forces HTTP instead of HTTPS.  So
far I've found plenty that will rewrite a URL, but they only rewrite the
address portion, and not the protocol.

John Lolis
Coordinator of Computer Systems

100 Martine Avenue
White Plains, NY  10601
tel: 1.914.422.1497
fax: 1.914.422.1452

https://whiteplainslibrary.org/

*“I would rather have questions that can’t be answered than answers that
can’t be questioned.”*
— Richard Feynman
<https://click.fourhourmail.com/5qure95xkf7hvvo93wh2/7qh7h8h05vr4zrtz/aHR0cHM6Ly9lbi53aWtpcGVkaWEub3JnL3dpa2kvUmljaGFyZF9GZXlubWFu>,
theoretical physicist and recipient of the Nobel Prize in Physics in 1965