Improve error message when no `di.yml` is found, or `di.yml` is empty

[tcwalther]

Currently, when running without a di.yml file, one gets the following error. Since this will be a common thing, the output should probably be a bit more user-friendly.

$> di build                                                                                                               ⏎
Traceback (most recent call last):
  File "/Users/tw/miniconda3/bin/di", line 11, in <module>
    load_entry_point('docker-interface==0.2.6', 'console_scripts', 'di')()
  File "/Users/tw/miniconda3/lib/python3.5/site-packages/docker_interface/cli.py", line 33, in entry_point
    configuration = base.apply(configuration, None, args)
  File "/Users/tw/miniconda3/lib/python3.5/site-packages/docker_interface/plugins/base.py", line 246, in apply
    "missing configuration; could not find configuration file '%s'" % args.file)
FileNotFoundError: missing configuration; could not find configuration file 'di.yml'

Equally, when the di.yml is empty, the output currently is:

$> di build                                                                                                                 ⏎
Traceback (most recent call last):
  File "/Users/tw/miniconda3/bin/di", line 11, in <module>
    load_entry_point('docker-interface==0.2.7', 'console_scripts', 'di')()
  File "/Users/tw/miniconda3/lib/python3.5/site-packages/docker_interface/cli.py", line 33, in entry_point
    configuration = base.apply(configuration, None, args)
  File "/Users/tw/miniconda3/lib/python3.5/site-packages/docker_interface/plugins/base.py", line 247, in apply
    configuration['workspace'] = os.path.join(dirname, configuration.get('workspace', '.'))
AttributeError: 'NoneType' object has no attribute 'get'

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