wrap all API calls with error catching, so all API return Json-based errors
update video_download
Merge branch 'develop' of github.com:learningequality/ka-lite into safe_api_errors

Conflicts:
	kalite/securesync/api_views.py
updating formatting on video_download
making strings more i18n-friendly, being careful to only display a single error of a single type.
fixed bug in display of API errors in video_player.js, safer error handing for 403 error in video_download.
changing _new to _blank
Merge pull request #458 from bcipolli/api_error_cleaning

Two simple "API error" changes