Bug#870814: show message if unable to play video

[積丹尼 Dan Jacobson]

Currently going to
$ /usr/lib/x86_64-linux-gnu/webkit2gtk-4.0/MiniBrowser https://www.youtube.com/
and trying to play a video now shows a message with a link to
https://www.youtube.com/supported_browsers
so I suppose this is an improvement.