[Feat] Placeholder Page when A Resource is Down

[didotb]

Summary

A custom / editable placeholder page for when a resource is down.

Motivation

Most of my clients are non tech savvy people who happen to not understand nor even see what Pangolin's reply is.
A really small "Gateway Unavailable" or "404 page not found" is something they don't understand and they often think it's a problem on their end, therefore, causes delays from being reported.

Proposed Solution

A select-able preset or a customizable page for each error that Pangolin might show.

Alternatives Considered

A global placeholder with better visibility for when an error is shown.

Additional Context

A perfect example would be Cloudflare's 103x error pages.

[didotb]

Unfortunately didn't see https://github.com/orgs/fosrl/discussions/658
Closed as duplicate of #658