Pass error through to the callback

With this change `atomify-css` passes the error through to the callback if an error occurred (instead of throwing it synchronously). It also makes the callback always be called asynchronously, instead [releasing zalgo](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony).

My use case is that a web server (even a development server) should that compiles code in-process should not have an exception thrown when a callback is provided. Currently it is unintuitive, since you have to wrap the code that uses a callback with a `try ... catch` block to catch the errors that occur when compiling.

Not sure how the built-in LESS and SASS compilers handle errors; I didn't check them.