rds_enhanced_monitoring/lambda_function.py could use more print statements for errors

https://github.com/DataDog/datadog-serverless-functions/blob/master/aws/rds_enhanced_monitoring/lambda_function.py

Actual: errors are displayed in the AWS console monitoring for the lambda. However, there are no CloudWatch logs.

Expected: errors are logged so that they can be inspected in the CloudWatch logs etc...

Thanks.  If folks have suggestions I can try to put together a PR.  Cheers.