Catch attempts to download or delete without selecting any collections

Fixes #552. Also fixes a bug a flash of type 'warning' was created. 'warning' is not a flash type and would not get displayed in the template
There is a `.warning` class in styles.css, and the template code in `journalist_templates/flashed.html` seems like it would show a flashed message with that class (it only filters out `banner-warning`). Were you not seeing flashed messages when the category was set to `warning`?