Fail gracefully if no bundles are defined yet in assets.yml #23
Description
From this Twitter thread: https://twitter.com/jcoglan/status/102459518188920833
If neither include_css nor include_js are being used to load bundles yet, AssetHat should at most log a warning if assets.yml is missing or has no bundles.
If include_css or include_js are being used to load bundles, and assets.yml is missing or has no bundles, AssetHat should raise an error.