> 1) A multiprocessing worker can not return (return, not raise!) an
> Exception. (...) raise an error if multiple arguments are required:
> TypeError: ('__init__() takes exactly 2 arguments (1 given)', 
> <class '__main__.MyException'>, ())

This problem comes from pickle, not multiprocessing: issue #1692335.

> 2) A multiprocessing worker can not return an hashlib Object.
> If this is attempted, pickle returns an error:

It is related to pickle and hashlib: #11771