[manifest] Graceful handling of no network connectivity in hosted apps (#339)

Would be great to have a mechanism to gracefully handle the case when a hosted app is launched which requires a network connection to function and the `start_url` fails to load within a configured timeout interval. If the OS knows up front there is no network connectivity and the app has signaled that a network connection is required, then don't bother attempting to load the `start_url` at all. 

Perhaps the manifest supports a `network_unavailable_url` member which is loaded in this scenario. The OS would need to cache this URL each time the app does load successfully (or at least every once in a while to pickup changes) to ensure it is available locally for future launch failures. If no `network_unavailable_url` is specified, then some sensible default "This app requires network connectivity" message would be rendered instead.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/339