Display a warning message in the download confirmation overlay when no file is available.

Currently the download overlay links to a file, even if it is `None`. With this PR the overlay displays a different warning message in that case.

![screen shot 2014-09-22 at 4 56 15 pm](https://cloud.githubusercontent.com/assets/736583/4358300/f9de4f6a-4268-11e4-836b-fb5fdf51a8b1.png)