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 general error object

I would say it's the "base" object for all native js errors. All other native js error objects inherit from this, so all other native js error objects pass the `instanceof Error` test.

>  -- it imparts no meaning to the situation that generated the error.

Subjective. Remove.