Added Exception to for describe_container_instances to match aws api

moto/ecs/models.py

@@ -790,6 +790,8 @@ class EC2ContainerServiceBackend(BaseBackend):
         cluster_name = cluster_str.split('/')[-1]
         if cluster_name not in self.clusters:
             raise Exception("{0} is not a cluster".format(cluster_name))
+        if not list_container_instance_ids:
+            raise JsonRESTError('InvalidParameterException', 'Container instance cannot be empty')
         failures = []
         container_instance_objects = []
         for container_instance_id in list_container_instance_ids:

tests/test_ecs/test_ecs_boto3.py

@@ -889,6 +889,12 @@ def test_describe_container_instances():
         instance.keys().should.contain('runningTasksCount')
         instance.keys().should.contain('pendingTasksCount')
 
+    with assert_raises(ClientError) as e:
+        ecs_client.describe_container_instances(
+            cluster=test_cluster_name,
+            containerInstances=[]
+        )
+
 
 @mock_ec2
 @mock_ecs