Better surfacing of underlying Redis connection issues

[svix-james]

We've noticed that we don't get meaningful errors when we have underlying connectivity issues to redis instances, e.g., when we're trying to connect to a TLS-enabled cluster with a non-TLS connection. Rather than getting an obvious error, we only see an eventual timeout in trying to get a connection from the underlying connection pool, i.e.:

thread 'main' panicked at 'Error retreiving connection from Redis pool: TimedOut'

Testing directly against a redis::cluster::ClusterConnection instance, we don't have the same problem. Connection errors are immediately surfaced (although the errors in the redis-rs lib are not great here either):

Failed to create initial connections- IoError.

I'm not immediately sure if there's a clean fix here on our end. It could be related to this: djc/bb8#141