Added error views for when data isn't available in the views.

![engagement-content](https://cloud.githubusercontent.com/assets/5265058/4279970/1b883db6-3d27-11e4-8753-9e951e856073.png)
![enrollment-activity](https://cloud.githubusercontent.com/assets/5265058/4279969/1b874e06-3d27-11e4-9d13-38c780bd2b47.png)
![enrollment-geography](https://cloud.githubusercontent.com/assets/5265058/4279968/1b86e4a2-3d27-11e4-93d7-47cd06a13e6b.png)

@rocha @clintonb 
By mocking `is_valid_course` you are mocking as little as possible. Mocking `requests` goes too deep into the implementation details which are not pertinent to the view being tested. The view should only care about the return value of `is_valid_course`.