CLI hangs if Docker isn't running

I opened https://github.com/microsoft/vscode-remote-release/issues/8801 and thought the dev container CLI overall wasn't working at the moment for me since commands kept hanging. I realized the Docker engine was paused on my machine, and once I started running it, the CLI works.

It'd be really helpful to notify users Docker isn't running or installed on their system, rather than silently hanging.