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 string representing the message the error was instantiated with.

A string of the value passed to `Error()` upon instantiation.

I say "value" because anything can be passed to `new Error()` but it will be converted to a string representation.

> Intended for human consumption.

Subjective. Remove.

> The message will appear in the first line of the stack trace of the error.

Make a note that while `error.message` can be assigned after the error has been instantiated, starting in v0.11 the reassigned message will not appear in `error.stack`. Only the original value passed to `new Error()`. The reason is because of V8 implementation details.