I don't really know where this belongs, but it is at least for me not an expected behaviour. It is weird for me at all. Please take a look if this is an intended behaviour and why it is like that.

Tested with py3.7.3, py3.8.0b1
```
A process in the process pool was terminated abruptly while the future was running or pending.
```

Tested with py 3.6.8:
```
Exception in thread Thread-1:
Traceback (most recent call last):
  File "C:\Python36\lib\threading.py", line 916, in _bootstrap_inner
    self.run()
  File "C:\Python36\lib\threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Python36\lib\concurrent\futures\process.py", line 272, in _queue_management_worker
    result_item = reader.recv()
  File "C:\Python36\lib\multiprocessing\connection.py", line 251, in recv
    return _ForkingPickler.loads(buf.getbuffer())
TypeError: __init__() missing 1 required positional argument: 'num'
```

I expect that the PoolBreaker exception would work too.

Code to test:
As attachment.

That's a super interesting bug! It looks like this issue is that your exception can't be round-tripped using pickle i.e.

>>> class PoolBreaker(Exception):
...     def __init__(self, num):
...         super().__init__()
...         self.num = num
... 
>>> import pickle
>>> pickle.loads(pickle.dumps(PoolBreaker(5)))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'num'