Having no background script should raise a warning if the app is launched from a local path

[m-alger]

Say we have an app which has a manifest member like the following. I think that this means it's a packaged app, but it's locally hosted.

 "app": {
    "launch": {
       "local_path": "index.html"
    }
 }

Caterpillar can still convert these to websites, and give a useful report, so we should change the current behaviour of raising an error on no background script to raising a warning if the app is launched from a local path. Note that this breaks the assumption that a background script exists, so part of the conversion process will also have to be updated.