PWA: Handle no network connectivity

[luhmann]

Find a suitable behavior for the app when there is no network connection.

[luhmann]

There is one case where this is more than "nice-to-have":

Steps to reproduce:

• A device was online and the user is experiencing the app
• He locks the device
• He reactivates the devices and network-access is not immediately available (e.g. searching for wifi)
• A scheduled, non-user-initiated network-request occurs

Actual behavior

• The devices immediately displays an error screen, cause the network connection could not be established

Expected behavior

• The app should detect a network error which suggests that it might be temporary and suspend going into an error state until a user-initiated-action occurs.
• Additionally some kind of retry might be possible

[luhmann]

Consider also creating a retry logic, for status codes > 500 ... issue is likely temporarily and should not require user interaction.