serve: Make some error conditions non-fatal, test them.

We don't want flexnbd serve to fall over and die if the client sends an invalid request.

tests/acceptance/fakes/source/connect_from_banned_ip.rb

@@ -13,10 +13,8 @@ addr, port = *ARGV
 
 client = FakeSource.new( addr, port, "Timed out connecting", "127.0.0.6" )
 sleep( 0.25 )
-client.ensure_disconnected
 
+rsp = client.disconnected? ? 0 : 1
 client.close
-exit(0)
-
-
+exit(rsp)