Media insertion error messages

This PR handles VE-600, VE-122 and part of VE-166.  It adds error messages when a user tries to add a video when they are not logged in or don't have permission, and when the video is non-premium and the wiki doesn't allow it. There's also a catch-all error when a url isn't supported or something went wrong with adding the video.

Right now this only covers videos, images are still handled with alert messages.