replace the standard 404 / 403 responses with plugins

usage

load plugins either globally with --on404 ~/dev/copyparty/bin/handlers/sorry.py or for a specific volume with :c,on404=~/handlers/sorry.py

api

each plugin must define a main() which takes 3 arguments;

cli is an instance of copyparty/httpcli.py (the monstrosity itself)
vn is the VFS which overlaps with the requested URL, and
rem is the URL remainder below the VFS mountpoint
- so vn.vpath + rem == cli.vpath == original request

examples

on404

sorry.py replies with a custom message instead of the usual 404
nooo.py replies with an endless noooooooooooooo
never404.py 100% guarantee that 404 will never be a thing again as it automatically creates dummy files whenever necessary
caching-proxy.py transforms copyparty into a squid/varnish knockoff

on403

ip-ok.py disables security checks if client-ip is 1.2.3.4

notes

on403 only works for trivial stuff (basic http access) since I haven't been able to think of any good usecases for it (was just easy to add while doing on404)