Feature: Bi-Direction friendly error names and statusCodes

I wanted to convert error codes I was getting from an external service, and your lib looked like a great starting point. Unfortunately, it only went from friendly names to status codes. So I added the other mapping.

I think I made all the appropriate changes to add this feature. Unfortunately it does change the API of the entire lib, but I hope you can forgive that.

I also noticed that you seem to dislike semicolons. So I guess I'm sorry that I included them in index.js