Fixes #283. If the mkdocs.yml is completely empty there is a traceback #288
Looks like a decent start. I'd love to see some tests for this and it would be good idea to handle other related problems at the same time. The YAML may not be empty, but could contain a totally invalid data structure.
It might be best to do something like:
with open(filename, 'r') as fp: user_config = yaml.load(fp) if not isinstance(user_config, dict): raise ConfigurationError("The mkdocs.yml file is invalid. See http://www.mkdocs.org/user-guide/configuration/ for more information.")