Give the user a better error message (without a stack trace) when there is a yaml error.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

compose/config/config.py

@@ -610,5 +610,6 @@ def load_yaml(filename):
     try:
         with open(filename, 'r') as fh:
             return yaml.safe_load(fh)
-    except IOError as e:
-        raise ConfigurationError(six.text_type(e))
+    except (IOError, yaml.YAMLError) as e:
+        error_name = getattr(e, '__module__', '') + '.' + e.__class__.__name__
+        raise ConfigurationError(u"{}: {}".format(error_name, e))