wrap all API calls with error catching, so all API return Json-based errors
update video_download
raise Http404, not return
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.
removing debug-only text.
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into safe_api_errors

Conflicts:
	kalite/templates/video_download.html
Merge pull request #439 from bcipolli/safe_api_errors

Safe api errors
Resolved merge conflicts, removed debug-only assignment, opened issue #461
changing _new to _blank
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into api_error_cleaning
fixing bug when json response returns empty.
Merge pull request #436 from bcipolli/raise_not_return

raise Http404, not return
Upgrading to video.js version 4, to get it working on IE.
Merge remote-tracking branch 'upstream/release-0.10' into ie_fixes
Use video.js 4 in IE, and video.js 3 in other browsers.