Fix the label selector to selector conversion error string.

The message as it is framed right now does not make any sense for the
end users of our system. It might even lead to confusion. So this is
attempt to make the error message less confusing.

pkg/apis/extensions/helpers.go

@@ -28,7 +28,7 @@ package extensions
 // func ScaleFromDeployment(deployment *Deployment) (*Scale, error) {
 // 	selector, err := unversioned.LabelSelectorAsSelector(deployment.Spec.Selector)
 // 	if err != nil {
-// 		return nil, fmt.Errorf("failed to convert label selector to selector: %v", err)
+// 		return nil, fmt.Errorf("invalid label selector: %v", err)
 // 	}
 // 	return &Scale{
 // 		ObjectMeta: api.ObjectMeta{