Provide a negative scenario test coverage for ReservationBuilder Validation

[maghamed]

Provide a negative scenario test coverage (Unit Tests) for ReservationBuilder Validation.

Need to cover cases when the ValidationException is thrown.

Currently, it's blocked by issue #81
Here is the explanation - https://github.com/magento-engcom/magento2/pull/64#discussion_r137484566