Tor should try harder to choose nodes in small networks

In https://trac.torproject.org/projects/tor/ticket/14881#comment:58 , pastly reports that in networks without bandwidth weights, tor says:

Oct 26 19:56:00.000 [warn] Failed to find node for hop 1 of our path. Discarding this circuit.

We should fix this at some point by falling back to choosing a random node.