Support throwing exceptions back into the caller

This allows a user of an actor_function to get errors by way of an exception.

Note that this isn't really practical for actor_methods
One way to model a traditional accounts

This example is based on comments in "Sane concurrency in Go", and
rewriting it based on how the code should work. Worth noting that this
shows how to do threadsafe exceptions *between* threads.

Unlike the code / example here:

   https://blog.mozilla.org/services/2014/03/12/sane-concurrency-with-go/

I feel that this example is somewhat more readable, while being just as safe.
Changed some names
Split the cash transfer examples into 3 examples

These are to be used in an upcoming blog post