unwrap ExecutionException to improve error reporting

e.g. instead of

    SQLActionException[Failed execution]

the user will now get

    SQLActionException[com.amazonaws.services.s3.model.AmazonS3Exception:
    Status Code: 403, AWS Service: Amazon S3, AWS Request ID:
    6F27AB0AB5FE3155, AWS Error Code: SignatureDoesNotMatch ...