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)

I talked to @shnayder and his observation is that for configurable messages, which are edx.org specific, it is ok if we don't have translations for now.

My opinion is that if we find a easy way to make this configurable without re-deployments, then lets go for it. Otherwise, lets just leave it as a django setting and we will make the changes later.