errorWidget should be called errorWidgetBuilder

`placeholder` reads like it expects a widget, `errorWidget` reeeally sound like I should provide a `Widget`, but no, they are both `Builders`, so they should be renamed to `placeholderBuilder` and `errorBuilder`, or there should be `placeholder` and `errorWidget` that accepts a `Widget` and leave the other to be builders, the way it is right now is confusing.

Also, `errorWidget` is of type `LoadingErrorWidgetBuilder`, Loading is just unnecessary, and the `placeholder` type doesn't have a "Loading" on its name, which is inconsistent.