[skip e2e]Add comment for msgIndexNodeIsUnhealthy (#15142)

Signed-off-by: cai.zhang <cai.zhang@zilliz.com>

internal/indexnode/errors.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 )
 
+// msgIndexNodeIsUnhealthy return a message tha IndexNode is not healthy.
 func msgIndexNodeIsUnhealthy(nodeID UniqueID) string {
 	return fmt.Sprintf("index node %d is not ready", nodeID)
 }