Modify(RegistrationDetailsDTO): add validation messages to validation annotations.
Refactor(AccountRequestErrorInterceptor): rename to ErrorInterceptor.
Modify(ErrorInterceptor): reply with ValidationInfoDTOs instead of just strings; add catch clause for UserNameExistsException.

Remove(RegistrationErrorInterceptor).
Modify(UserRegistrationServiceTest): add assertions that the expected reply is non null to the failure test cases.
Modify(ErrorInterceptor): add handling of AccountDoesNotExistException, UnavailableRequestTypeException and InvalidRequestException.

Remove(AccountNotOwnedByUserException).