Found by actually running furtka serve and clicking through the new
Settings UI: /api/gateway/status crashed the whole request with a raw
PermissionError traceback because conf_path.is_file() doesn't swallow
every OSError (confirmed directly, not assumed) when the parent directory
isn't accessible. A read-only status check must degrade to "not
configured" instead of 500ing; set_tunnel_enabled() now raises a clean
WireguardError instead of leaking an unhandled exception.
Also gives wireguard.py an env-var-overridable default path
(FURTKA_WIREGUARD_CONF / default_conf_path()), matching every other
runtime path in the codebase, instead of a single hardcoded constant.
Robert Syrnicki
91f2d6304c