Better connection status checks and error reporting

[daviddeutsch]

Mostly:

• Don't mark a channel as private if it wasn't private before (maybe some kind of user interaction should make sure that we don't get false positives)
• Under no circumstances should an error block your sanegrid if you still have perfectly good stuff in localStorage
• The refresh button needs to reflect loading properly
• An error bar above the footer should inform about problems

[warcode]

Implemented https://github.com/jirikavi/AngularJS-Toaster for error toasts in my fork.

Can't be bothered cherrypicking into a pull request, so if you are interested you can check out the commit https://github.com/warcode/yt-sanegrid/commit/05c477b02eba8c694d64c4befff82c05abf01754