Inconvinient errors handling

In node.js first argument of any callback function represents an error and should follow some rules:

 * It should always be `null` if request completed successfully.
 * It should describe an error otherwise.

So, passing `200` is against node.js callbacks convention.

It's also [a good practice](http://www.devthought.com/2011/12/22/a-string-is-not-an-error/) to wrap all errors using `new Error` constructor in order to capture stacktrace.