Add logging of ReseponseError when doing the cluster initialization steps so the true error is visible in older python versions that do not merge and propegate the original exception automatically

Author: Grokzen

rediscluster/nodemanager.py

@@ -228,6 +228,8 @@ def initialize(self):
             except (ConnectionError, TimeoutError):
                 continue
             except ResponseError as e:
+                log.exception("ReseponseError sending 'cluster slots' to redis server")
+
                 # Isn't a cluster connection, so it won't parse these exceptions automatically
                 message = e.__str__()
                 if 'CLUSTERDOWN' in message or 'MASTERDOWN' in message: