Need better error message when missing credentials

[ryanewel]

OS: Ubuntu 16.04
ROS Distro: Kinetic

When running roslaunch tts sample_application.launch without credentials I get the following error message:

Traceback (most recent call last):
  File "/ws/polly_test_ws/install/tts/lib/tts/polly_node.py", line 19, in <module>
    tts.amazonpolly.main()
  File "/ws/polly_test_ws/install/tts/lib/python2.7/dist-packages/tts/amazonpolly.py", line 424, in main
    AmazonPolly().start(node_name=node_name, service_name=service_name)
  File "/ws/polly_test_ws/install/tts/lib/python2.7/dist-packages/tts/amazonpolly.py", line 186, in __init__
    self.polly = self._get_polly_client(aws_access_key_id, aws_secret_access_key, aws_session_token, region_name)
  File "/ws/polly_test_ws/install/tts/lib/python2.7/dist-packages/tts/amazonpolly.py", line 217, in _get_polly_client
    return session.client("polly")
  File "/usr/lib/python2.7/dist-packages/boto3/session.py", line 263, in client
    aws_session_token=aws_session_token, config=config)
  File "/usr/lib/python2.7/dist-packages/botocore/session.py", line 850, in create_client
    credentials = self.get_credentials()
  File "/usr/lib/python2.7/dist-packages/botocore/session.py", line 474, in get_credentials
    'credential_provider').load_credentials()
  File "/usr/lib/python2.7/dist-packages/botocore/credentials.py", line 1662, in load_credentials
    creds = provider.load()
  File "/ws/polly_test_ws/install/tts/lib/python2.7/dist-packages/tts/amazonpolly.py", line 92, in load
    'aws-iot-with-certificate'
  File "/usr/lib/python2.7/dist-packages/botocore/credentials.py", line 308, in create_from_metadata
    access_key=metadata['access_key'],
TypeError: 'NoneType' object has no attribute '__getitem__'
[polly_node-2] process has died [pid 4631, exit code 1, cmd /ws/polly_test_ws/install/tts/lib/tts/polly_node.py __name:=polly_node __log:=/root/.ros/log/173ce162-be0f-11e9-b033-0242ac110003/polly_node-2.log].
log file: /root/.ros/log/173ce162-be0f-11e9-b033-0242ac110003/polly_node-2*.log

It looks like this is caused by missing credentials (and is solved by configuring my AWS credentials). That's not immediately obvious from the error message.