editBook should check if ISBN has been used by another book

[spaceninja]

In addBook, we can just check if there's already a book using this ISBN in allBooks, but in editBooks this won't work because the book we're editing is already using it! However, this is still a useful check in case the user changes the ISBN to one that's already being used. So we should still check, but the check needs to see if a book is already using this ISBN that has a different ID.