Add a friendly message if docker is not installed

[tadeugr]

Motivation

Currently if docker is not installed or not running, the following error is displayed.

Traceback (most recent call last):
  File "/usr/local/bin/dugaire", line 5, in <module>
    from dugaire.dugaire import main
  File "/usr/local/lib/python3.8/dist-packages/dugaire/dugaire.py", line 24, in <module>
    from pkg.docker import docker
  File "/usr/local/lib/python3.8/dist-packages/dugaire/pkg/docker/docker.py", line 18, in <module>
    _client = docker.from_env()
  File "/usr/local/lib/python3.8/dist-packages/docker/client.py", line 96, in from_env
    return cls(
  File "/usr/local/lib/python3.8/dist-packages/docker/client.py", line 45, in __init__
    self.api = APIClient(*args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/docker/api/client.py", line 197, in __init__
    self._version = self._retrieve_server_version()
  File "/usr/local/lib/python3.8/dist-packages/docker/api/client.py", line 221, in _retrieve_server_version
    raise DockerException(
docker.errors.DockerException: Error while fetching server API version: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

Add a more friendly error message.