Improved DynamoDB retry support 

This improves the information provided to the retry method. Most importantly, it makes it possible to determine the DynamoDB assigned RequestId, which you will need if you want to contact AWS support with an issue with a specific request. It also makes it possible to implement more complete logging on failed requests.
This change is fully backwards compatible, so old style retry methods will still work and the default error returns have not been changed. Unfortunately this means that clients will need to implement a retry method explicitly if they want access to the RequestId. A sample method is provided in the doc.