Explain when no transports can be used for adding a contact
When adding a contact in person, there are various circumstances where we can tell as soon as we've scanned the contact's QR code that we won't be able to communicate. For example:
- Both devices support Bluetooth, but neither device knows its own Bluetooth address (Android 8, see #1147)
- Both devices support wifi but they're on different networks (see #1223, #1178 (closed))
- One device only supports Bluetooth, the other only supports wifi If the user declines to activate Bluetooth and there's no wifi connection, we can tell even sooner that we won't be able to communicate.
We should detect these situations and explain the problem to the user in an actionable way.
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