fix import path

Importing .styl files with `@import` wasn't working for me.

I set 'filename' to the absolute path per [the docs](https://github.com/LearnBoost/stylus/blob/master/docs/import.md#stylus-import) and added a test case. 

I'm also throwing errors in the render callback (previously it was swallowing the error, causing component to fail with an unhelpful message about not finding main.css).