Document filter message for no results

www.agileplannerapp.com/boards/173808/cards/8672
addresses: https://govuk.zendesk.com/tickets/869065

In its current version the document filter shows a loader image when the search is being performed and doesn't provide any feedback if results are not found.

Editors can get confused if they're searching for a publication to link to a Statistics Announcement and no results are returned. It may seem as if the filter is not functioning.

I've added a suitable message which reminds editors that the lookup is scoped by publication type which is same as the type of the statistics announcement, and, publication state being not published.

![screen shot 2014-11-17 at 14 22 58](https://cloud.githubusercontent.com/assets/230074/5067284/3a7b3582-6e66-11e4-812c-76cc6950220e.png)