Empty configuration file raises AssertionError

[srak289]

Pointing carthage-runner to empty config.yml raises

Traceback (most recent call last):
  File "/root/carthage/bin/carthage-runner", line 192, in <module>
    args, unknown = carthage.utils.carthage_main_setup(parser, unknown_ok=True)
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/carthage/carthage/utils.py", line 285, in carthage_main_setup
    config.load_yaml(f, ignore_import_errors=ignore_import_errors)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/carthage/carthage/config/layout.py", line 94, in load_yaml
    assert isinstance(d, dict)
           ~~~~~~~~~~^^^^^^^^^
AssertionError

This could have a clearer message.