further enhance age-gate bypass

further enhance age-gate bypass

only non-embeddable, age-gated videos can't be played now. we try the
embedded version first, since this works on all videos that don't have
embedding disabled by the uploader. only on those, we fetch the /watch
version. if it still doesn't work, we look at the error message to
determine if this was an age-gate or another error.

further values for clientScreen (extracted from player/base.js):
	EMBED,WATCH,CHANNEL,LIVE_MONITOR,WATCH_FULL_SCREEN,ADUNIT,UNKNOWN