[PD-834] Show error when site package is missing.

Can't remember what the text is supposed to be for the error. 
Can we confirm it's a site package that is available for the user and for the current site as well? Right now that's a combination of the `user_available` and `filter_by_sites([current_site])` querysets. Not sure if it would be worth creating a third that just chains the two right away.