docker driver: improve error message when docker is not running

[medyagh]

when docker is not running at all :

$ ./out/minikube start --vm-driver=docker
😄  minikube v1.9.2 on Darwin 10.15.3
✨  Using the docker driver based on user configuration

❗  'docker' driver reported an issue: exit status 1
💡  Suggestion: Docker is not running or is responding too slow. Try: restarting docker desktop.

👍  Starting control plane node m01 in cluster minikube
🚜  Pulling base image ...
E0406 14:13:08.940391   44995 cache.go:114] Error downloading kic artifacts:  error loading image: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
🔥  Creating Kubernetes in docker container with (CPUs=2) (0 available), Memory=4000MB (0MB available) ...
🤦  StartHost failed, but will try again: creating host: create: creating: create kic node: creating volume for minikube container: output Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
: exit status 1
🤷  docker "minikube" container is missing, will recreate.

🔥  Creating Kubernetes in docker container with (CPUs=2) (0 available), Memory=4000MB (0MB available) ...

💣  Failed to start docker container. "minikube start" may fix it.: recreate: creating host: create: creating: create kic node: creating volume for minikube container: output Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
: exit status 1

😿  minikube is exiting due to an error. If the above message is not useful, open an issue:
👉  https://github.com/kubernetes/minikube/issues/new/choose