cmd/cli: add an internal warn level log writer

So important events like upstream online/offline/failed will be
preserved, and submitted to the server as necessary.

cmd/cli/prog.go

@@ -102,6 +102,7 @@ type prog struct {
 	queryFromSelfMap          sync.Map
 	initInternalLogWriterOnce sync.Once
 	internalLogWriter         *logWriter
+	internalWarnLogWriter     *logWriter
 	internalLogSent           time.Time
 	runningIface              string
 	requiredMultiNICsConfig   bool