[✨ feature] 502 error handling for caddy

[Fredkiss3]

Is your feature request related to a problem? Please describe.

We have an error page for when the page that a user tries to reach isn't available because the domain isn't associated to any service we get this nice page :

We need the same for when the upstream is unreachable, we can do that by adding a handler for 502 status codes (that's the status code that caddy sends).

---

• [x] Are you willing to make a PR for this ?

[Fredkiss3]

This could be interesting, but for later not now