docs: protected AmazonClientException unwrapExecutionException(ExecutionException e) {
          Throwable t = e;
          while (t.getCause() != null && t instanceof ExecutionException) {
              t = t.getCause();
          }
          if (t instanceof AmazonClientException) {
              return (AmazonClientException) t;
          }
          return new AmazonClientException("Unable to complete transfer: " + t.getMessage(), t);
      }