bugfix: deadlock

[SayreBlades]

notes

ughh, this was a nightmare:

1. gdb stacks did not indicate deadlock
2. It was actually an issue with aws. boto3 client was throwing an exception intermittently. These exceptions were being swallowed and the thread would be re-allocated to the pool.
3. bubbling up these exceptions to main thread is not documented anywhere in dask

red herrings:

boto/boto3#454
boto/boto3#801

canonical boto3 multithreading:

http://boto3.readthedocs.io/en/latest/guide/resources.html#multithreading-multiprocessing.

100 million prime numbers

I also dropped in a new prime number search example... shit is so hot:

$ AWS_PROFILE=personal AWS_DEFAULT_REGION=us-east-1 python examples/primes.py
[########################################] | 100% Completed | 43.7s
5761457

[SayreBlades]

This is the handler for exceptions that happen outside your ad-hoc client code. I.e. your client function may divide by zero, and that is handled by dask internal machinery execute_task (https://github.com/dask/dask/blob/master/dask/local.py#L280). However if there is an issue with boto3 or networking to the server, these errors also need to propogate up to the dask task executer.