Fix various reported video.js errors

We started seeing a handful of null reference javascript errors on NewRelic after last deploy. In experimentation, I was able to reproduce two JS error cases:

# Opening / closing modal window quickly

**Resolution**: Don't async add fallback player when modal is closed

- Also add more descriptive thrown error when fallback player addition fails (should not see further instances of)

# Tabbing between video tabs quickly

**Resolution**: Disable tracking video progress when video tab is hidden (see comment for details)


Not sure whether NR will bucket things if we have a non-static string.