Misconfigured plugin causes the app to fail

[clelland]

If the <preference> tag is not present in config.xml, or the named service worker script doesn't exist, then the fetch queue is never processed at all, and the application will white-screen on startup.

We need to handle this in a more robust manner. If the plugin cannot initialize properly, then it should not install the fetch delegate, and any serviceworker registration calls should return a rejected promise.