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).
> Subjective. Remove.

To rephrase this, the type of an `Error` instance imparts no special meaning, whereas `TypeError`, `RangeError`, `SyntaxError` (among other error subclasses) *do* imply a certain error-generating circumstance -- at least, that's the thought I'm attempting to convey here.