Create a more informative error message when the return string is larger than 4GiB

[crosenth]

Process ForkPoolWorker-2:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/multiprocessing/pool.py", line 125, in worker
put((job, i, result))
File "/usr/local/lib/python3.6/multiprocessing/queues.py", line 341, in put
obj = _ForkingPickler.dumps(obj)
File "/usr/local/lib/python3.6/multiprocessing/reduction.py", line 51, in dumps
cls(buf, protocol).dump(obj)
OverflowError: cannot serialize a string larger than 4GiB