Show error to tell developer they added a styleguide block for an undefined section

This pull request adds an error when a developer adds a section that isn't defined in css yet. I've had this question several times in my team. 

![Undefined styleguide element](https://f.cloud.github.com/assets/167882/117470/f30e44b6-6c3f-11e2-93f3-adc5928761cd.png)