Two simple "API error" changes

This depends on #439

1. video_player.js did not properly pass error information; any errors / info were not shown (including the warning when users watch videos while not logged in)
2. Safer handling of a 403 error during video download:
    If an admin was logged out (behind the scenes) while downloading videos on the update page, the page would be reloaded (with the assumption that the login page would be shown).  However, if the API returned 403 but the user was still logged in, this would cause a redirect loop.  In addition, all selections in the topic tree would be lost on refresh.   
    Now, a message (with a link to login) will be shown, instead of refreshing.  This should be clearer to users, and they won't be suddenly transported to another page (without a clear reason).