Empty configuration file raises AssertionError

Pointing carthage-runner to empty config.yml raises
```
Traceback (most recent call last):
File "/root/carthage/bin/carthage-runner", line 192, in
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.

Start with carthage/config/layout.py at load_yaml, then reproduce the failure by pointing carthage-runner at an empty config.yml. Replace the AssertionError outcome with a clearer configuration error, and verify that the empty-file invocation reports the new message.

Written by the indexing model from the issue text.