Chore: Add category error messaging, hide empty ones

This hides the category displays on the home page if there aren't any listings to display in a given category and provides a message if you go to a category page and there are no listings to display.

@cherimarie, would you mind testing this out for me? Still having trouble with the listing approval process, so I can't verify that the homepage hiding behavior doesn't just hide **all** listings **all** the time. :laughing: 

/cc @cherimarie @eastridge