Automatically switch to offline mode when manifest can't be signed

[fson]

When not in offline mode, Expo CLI currently needs to make a request to exp.host to to be able to sign a manifest. If making the request fails (e.g. no internet connection), we should automatically fall back to offline mode and notify the user about it.

Your internet connection has been lost, so Expo CLI has switched to offline mode. For security reasons there are several limitations in offline mode, read more at: https://docs.expo.io/versions/v31.0.0/workflow/expo-cli-offline-mode_TBD