When a startup step failed (for example, the port was already in use), runNavidrome only logged the error and returned. In service mode, service.Run() kept waiting for a stop signal, so the process stayed up serving nothing and the service manager never restarted it. A plain run exited with code 0.
runNavidrome now returns the error, unless its context was cancelled by a normal shutdown. Both the plain run and the service goroutine exit with code 1 on that error. The systemd unit no longer lists 1, 2 and 8 in SuccessExitStatus, so Restart=on-failure restarts the service on exit code 1.
Fixes#6235