rbd: check stdErr for does not have a parent error

actual error will be present in the stdErr not the error
when we try to add a task to flatten the rbd image. This
commits corrects the error checking when the image does
not have a parent.

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

internal/rbd/rbd_util.go

@@ -471,6 +471,9 @@ func addRbdManagerTask(ctx context.Context, pOpts *rbdVolume, arg []string) (boo
 			util.WarningLog(ctx, "uncaught error while scheduling a task (%v): %s", err, stderr)
 		}
 	}
+	if err != nil {
+		err = fmt.Errorf("%w. stdError:%s", err, stderr)
+	}
 	return supported, err
 }