Give a better error message when the user asks us to scan 192.168.x.y

We have a user setting up a bridge with port forwarding, and they were using the 'scan' tool to see if it's reachable, but it was saying that it's unreachable, with reason timeout -- and it turns out they were asking it about 192.168.1.53.

We should intercept requests to scan rfc1918 netblocks, and give users more actionable feedback, e.g. pointing them to something that teaches them about private addresses and why they need to be testing their external address here.