Improve error messages when nodes can't communicate with each other. (#223)

* Good error messages when nodes can't communicate with each other

* Print more information when starting the head node.

* Change retries back to 5.

python/ray/services.py

@@ -166,6 +166,7 @@ def wait_for_redis_to_start(redis_host, redis_port, num_retries=5):
   while counter < num_retries:
     try:
       # Run some random command and see if it worked.
+      print("Waiting for redis server at {}:{} to respond...".format(redis_host, redis_port))
       redis_client.client_list()
     except redis.ConnectionError as e:
       # Wait a little bit.

python/ray/worker.py

@@ -674,7 +674,9 @@ def get_address_info_from_redis(redis_address, node_ip_address, num_retries=5):
       if counter == num_retries:
         raise
       # Some of the information may not be in Redis yet, so wait a little bit.
-      print("Some processes that the driver needs to connect to have not registered with Redis, so retrying.")
+      print("Some processes that the driver needs to connect to have not "
+            "registered with Redis, so retrying. Have you run "
+            "./scripts/start_ray.sh on this node?")
       time.sleep(1)
     counter += 1