[skip ci] Add comment for errors in query node (#8417)

Signed-off-by: bigsheeper <yihao.dai@zilliz.com>

internal/querynode/errors.go

@@ -16,6 +16,7 @@ import (
 	"fmt"
 )
 
+// error msg of unhealthy query node
 func msgQueryNodeIsUnhealthy(nodeID UniqueID) string {
 	return fmt.Sprintf("query node %d is not ready", nodeID)
 }