Use DOCKER_HOST environment variable to find Docker daemon

Removed all "smart" connection logic. Fig either uses the DOCKER_HOST
environment variable if it's present, or passes `None` to docker-py,
which does the "right thing" (i.e. falls back to the Unix socket).

This means we no longer know at URL-deciding time whether we can connect
to the Docker daemon, so we wrap `dispatch` in a `try/except` which
catches `requests.exceptions.ConnectionError`.

fig/cli/command.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 from __future__ import absolute_import
 from ..packages.docker import Client
+from requests.exceptions import ConnectionError
 import errno
 import logging
 import os
@@ -11,12 +12,23 @@ from ..project import Project
 from .docopt_command import DocoptCommand
 from .formatter import Formatter
 from .utils import cached_property, docker_url
+from .errors import UserError
 
 log = logging.getLogger(__name__)
 
 class Command(DocoptCommand):
     base_dir = '.'
 
+    def dispatch(self, *args, **kwargs):
+        try:
+            super(Command, self).dispatch(*args, **kwargs)
+        except ConnectionError:
+            raise UserError("""
+Couldn't connect to Docker daemon at %s - is it running?
+
+If it's at a non-standard location, specify the URL with the DOCKER_HOST environment variable.
+""" % self.client.base_url)
+
     @cached_property
     def client(self):
         return Client(docker_url())