Clear error message when no bindable elements found. #75

Feature request

When creating a new view prefab, I made the mistake to put bindable elements on the root game object, and could not get anything to work. While going through the code I ended up seeing that bindable elements should be on child game objects only.

_bindableElements = RootElement
    .GetComponentsInChildren<IBindableElement>(true)
    .Where(element => ((MonoBehaviour) element).gameObject != RootElement)
    .ToList();

I suggest adding a debug message when the _bindableElements list has 0 elements such as :
"No bindable elements found, did you add bindable elements on children gameobjects of the Canvas view's Game object ?" Or something similar.