Handle AccessDenied

[alexcasalboni]

When the Lambda function doesn't have enough permissions to read the SSM parameter, you get an AccessDeniedException ("botocore.exceptions.ClientError: An error occurred (AccessDeniedException)"). We could handle that here.

This way we could show customers a proper error such as "Your Lambda function is missing the required IAM permissions to read the chaos configuration from SSM" - and maybe even provide them with the correct JSON document, as documented here.