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.

push: make pushkey part advisory The current behavior (with bundle1) is to let the rest of the push succeed if the pushkey call (phases, bookmarks) failed (this comes from the fact that each item is sent in its own command). We kept this behavior with bundle2, which is highly debatable, but let us keep thing as they are now as a start. We are about to enforce 'mandatory' pushkey part as 'mandatory' successful, so we need to marks parts as advisory to preserve the current (debatable) behavior.