Compile list of state-checks to short-circuit on, and then gate the various operations behind them

[StevenTammen]

Right now, especially for the larger scripts that call multiple functions, it is possible to "half-succeed", which will leave you in a state somewhere between where you started and what you wanted.

For example, currently build_video.py (the main compound script) might correctly process topic transition video segments, but then fail in the timestamps function if there is no playlist frontmatter variable in the content file. (Necessary for correctly building the YouTube description).

What would be more desirable is to make all scripts atomic, so that they either fully succeed or don't run at all (short-circuit upfront). This ought to be possible with a bit of thought and refactor -- checking the error cases upfront.