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 
Avoid including HTML in text marked for translation as it is possible for translators to break the tags. Try the following:

``` jinja
{% captureas link_start %}<a href="{% settings_value 'DOCUMENTATION_LOAD_ERROR_URL' %}">{% endcaptureas %}
{% captureas link_end %}</a>{% endcaptureas %}
{% blocktrans %}This data may not be available for your course.  You can {{ link_start }}view the full list{{ link_end }} of potential causes for these errors in the documentation.{% endblocktrans %}
```