legacy launcher will not attempt loading the gadget code if gadgetBaseUrl is empty

A use case in the Chrome app is that `gadgetBaseUrl` can be empty for legacy gadgets.

The current legacy-launcher will try loading the URL `./gadget.js` or `./gadget.css` and fail with an error message.

With this patch, there will be a console message about empty `gadgetBaseUrl` but no errors.
Please use `console.warn` instead of "Warning: ". Otherwise fine.