Do a reachability check before polling for clients

I'm looking at https://trac.torproject.org/projects/tor/attachment/ticket/29461/example_metrics.log and I've seen one TR=2 and another TR=5 record. I'm not aware of the current situation in TR but as far as I remember Tor was censored there (it is even stated in the Torlauncher).

We can have a snowflake do a reachability check of the bridge(s) and broker (and maybe Tor domains/dirauths) before trying to poll for clients. If this check fails, we should display a user-facing error message or warning and hold off on polling while the bridge is unreachable