Surface a user-visible error of YT onReady never fires

[calzoneman]

Occasionally, the YouTube Iframe API fails to load completely and never fires the onReady event (but still plays the current video since autoplay is turned on). This results in the player being broken as no sync operations can occur without onReady. This results in users being confused about why it doesn't work.

It should be straightforward to set a timeout for onReady, show an error explaining the player may experience problems, and suggest the troubleshooting steps to look for extensions that may be interfering with the YT API.

[calzoneman]

https://github.com/calzoneman/sync/wiki/Troubleshooting-common-issues-in-the-browser#youtube-does-not-sync--does-not-advance-to-the-next-video