This helper addresses a common pattern where one spawns N processes to
work on some common task (e.g. parallel preprocessing or multiple
training loops).

A straightforward approach is to use the multiprocessing API directly
and then consecutively call join on the resulting processes.

This pattern breaks down in the face of errors. If one of the
processes terminates with an exception or via some signal, and it is
not the first process that was launched, the join call on the first
process won't be affected. This helper seeks to solve this by waiting
on termination from any of the spawned processes. When any process
terminates with a non-zero exit status, it terminates the remaining
processes, and raises an exception in the parent process. If the
process terminated with an exception, it is propagated to the parent.
If the process terminated via a signal (e.g. SIGINT, SIGSEGV), this is
mentioned in the exception as well.

Requires Python >= 3.4.