Add an option for action to take when requested subtitles are unavailable

[pkkm]

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Description of your issue, suggested solution and other information

Subtitles are sometimes essential for a video to be useful. For example, a person learning Spanish may wish to download only the videos that contain both English and Spanish subtitles from a large playlist. The same applies to a machine learning researcher working on automatic translation or speech recognition.

I propose adding an option for this, for example --on-missing-sub warn|skip|abort. warn (the default) would be the current behavior, skip would skip downloading the video when any of the subtitles specified with --sub-lang are unavailable and abort would cause youtube-dl to quit.

An example invocation (for the use-case described in the first paragraph) would be:

youtube-dl --write-sub --sub-lang en,es-ES --on-missing-sub skip <URL of playlist>