Add HexDocs error page with CSS

This PR adds an error page for HexDocs (no homepage for the time being, but it's coming :smile:). It's built on Yahoo's Pure CSS framework, and the CSS totals 5.2KB minified (included).

If the user suspects there is an error, it directs the them to open an issue on Github, or email support.

Here's a screenshot:
![screen shot 2014-10-17 at 6 49 17 pm](https://cloud.githubusercontent.com/assets/860934/4675861/4c658768-55d2-11e4-9850-ab9206338e54.png)

Let me know what you think! Feedback would be greatly appreciated.

:bouquet: :cherry_blossom: