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).
I went with "Errors" here because the following section is an enumeration of the error types that JS itself will generate, divorced from the mechanism by which they are delivered to the user -- an exception being a combination of any of the Error types enumerated with the act of being thrown. I'll defer to @trevnorris on this, since he has stronger opinions on the usage of the word "exception", and I'm largely borrowing my definition from him.