Add asset_path helper to precompile example

This adds an asset_path helper and updates the error messaging in the precompile script.

I think it would be helpful if connect-mincer and/or mincer docs would include an example of using  the asset_path helper inside of a stylus file. I took me some time to translate the ejs example. Something as simple as this:

```css
background: url(asset_path('background.jpg'));
```