Ensure that session is rolled back on bulk creates

During bulk creates, the session is began explicitely;
ensure that it gets rolled back before re-raising in
order to avoid triggering InvalidRequestError
exceptions when the session is reused.

Partial-bug: 1244757

Change-Id: I797faeec2ca5374620db905599ab6e6f04fdcabd
Merge "Ensure that session is rolled back on bulk creates"