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

**Issue**: YouTube blocking detection returns later than the modal is closed, so the swap-out fails

**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

**Issue**: Hiding a not-completely-initialized video.js flash player causes an infinite interval error

![image](https://cloud.githubusercontent.com/assets/206973/3649841/362244f6-1116-11e4-9841-fb45b54ef31a.png)

**Resolution**: Disable tracking video progress when video tab is hidden (see comment for details)


Seems a little confusing that in the outer scope we use shouldStillAdd as a boolean, and now in this scope it's a function.