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