It should be a string, not a tuple, and it has a different form.
I'm not sure if it used to be different, but in the most recent
boto3/botocore, the message is "Stack with id {id} does not exist"
```python
>>> cf = boto3.client('cloudformation', region_name='us-west-2')
>>> try:
... cf.describe_stacks(StackName='adfgfhghg')
... except botocore.exceptions.ClientError as e:
... print e.response['Error']['Message']
...
Stack with id adfgfhghg does not exist
```
I am on boto3 1.4.2 and botocore 1.4.82 as of this commit message.
231d3cadcb