fix for cannot find ffmpeg

[erdkse]

[20lives]

Hi, Thank you for your work, I am not sure downloading FFmpeg every npx run is the best way, I will consider this.

[JonFranchi]

May I suggest it just check for ffmpeg and throw a helpful error if it isn't present? My impatient self never read the full readme and was decoding cryptic error messages, not having realized I needed to install something else.