detect missing nvidia driver and give a useful prompt

[tfoote]

If you don't install nvida-docker2 you get this less than helpful traceback.

Executing command: 
docker run --rm -it -v /home/yadu:/home/yadu   --gpus all -v /run/user/1000/pulse:/run/user/1000/pulse --device /dev/snd  -e PULSE_SERVER=unix:/run/user/1000/pulse/native -v /run/user/1000/pulse/native:/run/user/1000/pulse/native --group-add 29   -e DISPLAY -e TERM   -e QT_X11_NO_MITSHM=1   -e XAUTHORITY=/tmp/.docker9h4b_k2f.xauth -v /tmp/.docker9h4b_k2f.xauth:/tmp/.docker9h4b_k2f.xauth   -v /tmp/.X11-unix:/tmp/.X11-unix   -v /etc/localtime:/etc/localtime:ro  a8728a86bf55 
docker: Error response from daemon: could not select device driver "" with capabilities: [[gpu]].