Avoid connecting to a default node

[ben-kaufman]

If the app cannot manage to connect to one of the nodes from the configured nodes it should work in an "offline mode", it must not connect to a public node by default without the user explicitly allowing it.

In general, we should make sure the user has complete contol on the node connection. The Node Settings can suggest default nodes, but it shouldn't connect to them if they are not listed or were removed by the user.