Refactor error page css

A more _rails-like_ and _error-like_ UI. No more random orange and yellow color :wink: 

![capture decran 2014-01-28 a 13 53 14](https://f.cloud.github.com/assets/464900/2022426/772acff6-884d-11e3-9b00-9b185e77a797.png)