code: def get_security_error(node):
          
          assert node.Name == 'securityData' and not node.IsArray
          if node.HasElement('securityError'):
              secid = XmlHelper.get_child_value(node, 'security')
              err = XmlHelper.as_security_error(node.GetElement('securityError'), secid)
              return err
          else:
              return None
docs: return a SecurityError if the specified securityData element has one, else return None