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).
> System errors are errors that are generated in response to your program's runtime [...]

"System errors" already denotes it is an error.

No personal pronouns. Instead use a determiner.

> System errors are generated in response to a program's runtime [...]