Updated copy on section error to include student enrollment example.

@rocha @clintonb @lamagnifica 

![screen shot 2014-10-01 at 11 23 23 am](https://cloud.githubusercontent.com/assets/5265058/4477557/f7be1b30-497e-11e4-97c1-0bf878436820.png)

We have a similar problem with the announcements. If we make them configurable via database, they won't be picked up by the translators, unless we find some alternative.

On the other hand, hard coding it into the template does not guarantee that we will get a translation if we have to make a sudden change in its content.