Adding Validation to Book

Now that we've added validation to the Reader model, our next task is to add validation to Book. In this case we just want to make sure that we have an author and a title.

Book:
	- title: String
  * Check if title exists
	- author: String
  * Check if author exists
	- genre: String
	- ISBN: String

1. Make sure constraints and validation are added to the Book model.
2. Make sure the controller knows how to handle the different error messages the model might throw.
3. Write unit tests that make sure that this functionality is in place.