Show error page for apps that fail to boot

[dbackeus]

Pow used to have this once upon a time but puma-dev never implemented it: puma/puma-dev#124

What POW error pages looked like:

The main purpose is to help the developer fix whatever is preventing the app to boot, so inclusion of the error log is key.

[tcannonfodder]

@dbackeus I'm wondering if Caddy's handle_errors would be helpful here. https://caddyserver.com/docs/caddyfile/directives/handle_errors

It's important to note that caddy-dev doesn't do any process management or auto-booting apps (since it's just sending everything over to Caddy). So, really, the things to handle are:

• When the request goes through the Caddyfile and doesn't end up in an expected place (the unknown app error)
• When there's an error within Caddy itself, which is hopefully where handle_errors comes in