Creates an error execution reply message.
function createExecuteErrorResponseMessage(parentMessage, executionCount, error, traceback) {
  var content = {
    status: 'error',
    execution_count: executionCount,
    ename: error.constructor.name,
    evalue: error.toString(),
    traceback: traceback
  };

  return newMessage(_messageNames.executeResponse, parentMessage, content);
}