ytdl_hook: check youtube-dl version if it breaks

Some failures by youtube-dl prompt the user to submit a bug report.
If such a failure occurs, we can compare youtube-dl's version to the
current calendar date to see how old it is. We don't make this check
on every youtube-dl failure, as failing to extract an URL is quite
common, and waiting for a second blocking python interpreter startup
for every such case would be a bit unpleasant.

Here the assumption is made that any youtube-dl version older than
3 months is probably severely out of date. Users will be warned about
this.

We also output the trimmed stderr of youtube-dl with msg.error, as this
appeared to have been the behaviour of utils.subprocess without stderr
capturing. Since this uses mp.command_native now, we'll have to do this
ourselves where appropriate.

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