elm: print the error message inside elm_widget_sub_object_add().

elm: print the error message inside elm_widget_sub_object_add().

1. Do not need to print the same error message from all the widget codes.
2. Even though elm_widget_sub_object_add() can be used internally, there should be no error message at all.
   Elm devs should fix it beforehand.
   So it looks ok to print the error message in elm_widget_sub_object_add() to force elm devs to fix it.
3. Got additional code cleanups.