Add error message to ArgumentError when adding a new item to a loader and the keys 'id' and 'term' are not defined.

I think it is a good practice to add this message when the `ArgumentError` is raised.

I started playing around with this gem and I got an `ArgumentError` when trying to create a simple item in the `Loader`. The only way to realize what was missing was checking part in the documentation and fully in the `add` method in the `Loader` class. I hope this message helps someone else when adding items.