feat: move entity and usecase to root folder

entity/error.go

@@ -0,0 +1,21 @@
+package entity
+
+import "errors"
+
+//ErrNotFound not found
+var ErrNotFound = errors.New("Not found")
+
+//ErrInvalidEntity invalid entity
+var ErrInvalidEntity = errors.New("Invalid entity")
+
+//ErrCannotBeDeleted cannot be deleted
+var ErrCannotBeDeleted = errors.New("Cannot Be Deleted")
+
+//ErrNotEnoughBooks cannot borrow
+var ErrNotEnoughBooks = errors.New("Not enough books")
+
+//ErrBookAlreadyBorrowed cannot borrow
+var ErrBookAlreadyBorrowed = errors.New("Book already borrowed")
+
+//ErrBookNotBorrowed cannot return
+var ErrBookNotBorrowed = errors.New("Book not borrowed")