Expose transaction cancellation reasons

[ikonst]

This will allow the caller to know e.g. which of the transaction items ran into a conflict, which can sometimes allow app logic to better handle the error without needing to make more requests.

A transaction cancellation response has the following shape:

{
  "__type": "com.amazonaws.dynamodb.v20120810#TransactionCanceledException",
  "CancellationReasons": [
    {
      "Code": "None"
    },
    {
      "Code": "ConditionalCheckFailed",
      "Message": "The conditional request failed"
    }
  ],
  "Message": "Transaction cancelled, please refer cancellation reasons for specific reasons [None, ConditionalCheckFailed]"
}

where the first item was not a reason for cancellation and the second one was.

Curiously, this field is not parsed by botocore: only some of the AWS SDKs (Java, Ruby...) parse it.