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.
bundle2: add an error message to push race error
Errors with no explanations makes my uncle Bob sad.