httppeer: reintroduce _abort that accidentally was removed in 167047ba3cfa Including the missing test coverage that would have caught it.

bundle2: gracefully handle hook abort We make sure any exceptions raised during the whole span of handling bundle2 processing are decorated. This let us catch exceptions raised by hooks prior to transaction commit.

bundle2: gracefully handle PushRaced error during unbundle Same drill again. We catch the PushRaced error, check if it cames from a bundle2 processing, if so we turn it into a bundle2 with a part transporting error information to be reraised client side.