3-1-3-author-book excercise: added correct exception handling for testing of duplicate book ISBN

[JoshukLite]

In task 3-1-3-author-book it's not clearly mentioned that the Book entity should have @GeneratedValue annotation with default (AUTO) strategy. Also, there are no tests (AuthorBookMappingTest.java) where this strategy is checked.

The third test (saveBookWithDuplicateIsbn()) will work just fine if Book ID generation strategy is default (AUTO), but it won't pass the test if Book ID generation strategy value is IDENTITY. Furthermore, it's really unobvious why the test is failing, but ConstraintViolationException will be always there:

1. In case when Book ID generation strategy is the default (AUTO), hibernate will throw RollbackException, but its cause will be ConstraintViolationException.
2. In case the Book ID generation strategy is IDENTITY, hibernate will throw ConstraintViolationException.

From my point of view third test saveBookWithDuplicateIsbn() should check if any exception mentioned above is the exception thrown by entityManager.persist(...) method.