Improve error message when sudo is needed to run Docker

[l0rd]

SublimeDocker requires that the Docker can be run without using sudo.

If this requirement isn't satisfied, Sublime Text logs the following error:

FATA[0000] Get http:///var/run/docker.sock/v1.17/version: dial unix /var/run/docker.sock: permission denied. Are you trying to connect to a TLS-enabled daemon without TLS?

It shoud pop up a more explicit message instead:

You should add your user to the "docker" group with something like:
sudo usermod -aG docker' $your_user