Render a message in place of a graph when no data is available

Adds a "no data" message to graphs when collection fetching fails.

To test, go into a `Collection#parse` method (eg https://github.com/alphagov/spotlight/blob/master/app/common/collections/grouped_timeseries.js#L20) and hardcode `data` to `[]`.

Example from Carer's Allowance:

![screen shot 2014-04-10 at 15 43 34](https://cloud.githubusercontent.com/assets/117398/2669090/d29eae08-c0be-11e3-9218-87fb91f450d5.png)