doc: add documentation for errors

This adds documentation around what errors are generated by Node and how to intercept them.

The rendered docs are available [here](http://neversaw.us/nodejs-api/errors.html).
A JavaScript exception is _anything_ that's thrown. e.g. `throw 'this is a string';` is an exception. `Error()` just instantiates a new `Error` object instance. They are not the same.