bundle2: abort when a mandatory pushkey part fails So far, result of a pushkey operation had no consequence on the transaction (beside the change). We makes it respect the 'mandatory' flag of part so that failed pushkey call abort the whole transaction. This will allow rejecting changes (primary target: changesets) regarding phases or bookmark criteria in the future (when we will push such data in a mandatory part). We currently raise an abort error because all clients support it. We'll introduce a more precise error in the next changesets.