Attempt NAT traversal when fetching, even if server is inaccessible

[bschwind]

Scenario: You have machines all in one physical building on a LAN, all connected via innernet. Maybe each peer connects to each other peer's external endpoint. innernet-server runs in a separate network.

If the building's external internet connection goes down, but machines are still connected via the LAN, the peers within that LAN should all be able to talk to each other by running NAT discovery with endpoints they've cached from the innernet server. Currently peers will just lose connectivity to each other, because although they have the internal endpoint cached in /var/lib/innernet/network_name.json, they won't do anything with it if the connection to the server fails.

I think we just need to modify the fetch functionality here to run client NAT traversal in case the innernet server can't be contacted. Otherwise, the client fetch command fails and causes the binary to exit without attempting to reconnect to peers.