This endpoint is used to support Groupon initiated cancellations.

Upon cancellation, the state of the reservation should be transitioned to cancelled. This endpoint should behave in a transactional manner, that is, if any unit in the request can't be transitioned to cancelled the whole request should fail with error code UNIT_NOT_CANCELLABLE, and no state transition should happen to any of the units.

Note: Unit states changes might happen on multiple API calls.