Handle errors

[pfist]

Hiei should handle errors with a custom class that extends the native Error object. Read this article from Node Best Practices for guidance.

[pfist]

I opted not to use a custom error class because for v1, I only really needed it for one error (when the run method isn't implemented on a class). Seemed like a wasted effort, so I'll come back to this in the future when the need arises.