Changed the getInvalidated() method as Openmrs required to let the Boolean variable:invalidated show in REST resource display
Added ResourceDoesNotSupportOperationException(..), when delete a patient assignment, if the assignment does not exist or has already been invalidated, throw this exception with message says the assignment does not exist or has been invalidated already; when save a patient assignment, if the assignment exist already, throw this exception with message says that the assignment exist.