Confusing error message when master is not yet selected

[shtratos]

When I try to submit a job to a fresh cluster that still has no master selected I get this error:

$ aztk spark cluster submit --id testpy --name pipy2 ~/aztk/examples/src/main/python/pi.py 100
-------------------------------------------
Spark cluster id:        testpy
Spark app name:          pipy2
Wait for app completion: True
Application:             /Users/uuu/aztk/examples/src/main/python/pi.py
Application arguments:   ['100']
-------------------------------------------
Traceback (most recent call last):
  File "/Users/uuu/.virtualenvs/aztk/bin/aztk", line 11, in <module>
    load_entry_point('aztk', 'console_scripts', 'aztk')()
  File "/Users/uuu/aztk/aztk_cli/entrypoint.py", line 35, in main
    run_software(args)
  File "/Users/uuu/aztk/aztk_cli/entrypoint.py", line 63, in run_software
    func(args)
  File "/Users/uuu/aztk/aztk_cli/spark/endpoints/spark.py", line 31, in execute
    func(args)
  File "/Users/uuu/aztk/aztk_cli/spark/endpoints/cluster/cluster.py", line 81, in execute
    func(args)
  File "/Users/uuu/aztk/aztk_cli/spark/endpoints/cluster/cluster_submit.py", line 139, in execute
    wait=False
  File "/Users/uuu/aztk/aztk/spark/client.py", line 92, in submit
    cluster_submit_helper.submit_application(self, cluster_id, application, wait)
  File "/Users/uuu/aztk/aztk/spark/helpers/submit.py", line 117, in submit_application
    task = affinitize_task_to_master(spark_client, cluster_id, task)
  File "/Users/uuu/aztk/aztk/spark/helpers/submit.py", line 107, in affinitize_task_to_master
    master_node = spark_client.batch_client.compute_node.get(pool_id=cluster_id, node_id=cluster.master_node_id)
  File "/Users/uuu/.virtualenvs/aztk/lib/python3.6/site-packages/azure/batch/operations/compute_node_operations.py", line 368, in get
    'nodeId': self._serialize.url("node_id", node_id, 'str')
  File "/Users/uuu/.virtualenvs/aztk/lib/python3.6/site-packages/msrest/serialization.py", line 470, in url
    data = self.validate(data, name, required=True, **kwargs)
  File "/Users/uuu/.virtualenvs/aztk/lib/python3.6/site-packages/msrest/serialization.py", line 540, in validate
    raise ValidationError("required", name, True)
msrest.exceptions.ValidationError: Parameter 'node_id' can not be None.

It was quite hard to understand at first that this means "master is not selected yet"