Prevent actors from being placed on removed nodes or nodes with no CPUs. (#527)

* Make note about bug in which actor creation notification message is not received.

* Prevent actors from being created on removed nodes.

* Prevent actors from being created on nodes with no CPUs.

* Fix linting.

* Add test for scheduling actors on local schedulers with no CPUs.

* Improve error message when actors created before ray.init called.

test/actor_test.py

@@ -565,6 +565,19 @@ class ActorSchedulingProperties(unittest.TestCase):
 
 class ActorsOnMultipleNodes(unittest.TestCase):
 
+  def testActorsOnNodesWithNoCPUs(self):
+    ray.init(num_cpus=0)
+
+    @ray.actor
+    class Foo(object):
+      def __init__(self):
+        pass
+
+    with self.assertRaises(Exception):
+      Foo()
+
+    ray.worker.cleanup()
+
   def testActorLoadBalancing(self):
     num_local_schedulers = 3
     ray.worker._init(start_ray_local=True, num_workers=0,