[teachertube] Fix extraction for Python 3
[dreisat] Fix thumbnails' width and height
[dreisat] Modernize
[veoh] Capture error message
[nuvid] Improve extraction
[nuvid] Remove superfluous slash
rtmp and hls downloaders: Clarify error message when the external tools are not installed

Ask to install them, as we do in the postprocessor.
We get some reports with it, like #3061 or #3048.