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 
I tested this out, but it doesn't quite work.  The inclusion tag needs to decorate the public method, ex:

```
@register.inclusion_tag('section_error.html')
def show_metrics_error():
    return _section_error('metrics')


def _section_error(content_type):
    return {'content_type': content_type}
```

Otherwise a block error is thrown.  It doesn't see worth it in this case.