_configure_logging opened InodeAwareRotatingFileHandler against
DECNET_SYSTEM_LOGS (default: relative decnet.system.log) without
guarding OSError. Under systemd with ProtectSystem=full +
ProtectHome=read-only and no writable path baked into the unit, the
first import of decnet.config raised OSError and the daemon died
before it could even print a useful error — the root-cause log line
showed up in journalctl as a stack trace rather than a warning.
Wrap the handler attachment in try/except OSError and log a single
WARNING via the already-installed stream handler. stderr is always
attached, so losing the file handler means operators tail
journalctl / docker logs instead — the daemon keeps running.