When a worker process terminates unexpectedly,
:class:`concurrent.futures.ProcessPoolExecutor` now sets a separate
:exc:`~concurrent.futures.process.BrokenProcessPool` exception on each pending
future instead of sharing a single instance among them all.  Sharing one
exception produced malformed tracebacks: each
:meth:`Future.result() <concurrent.futures.Future.result>` call re-raised the
same object, appending another copy of the traceback to it.
