code: func GetCustomFieldsManager(c *vim25.Client) (*CustomFieldsManager, error) {
  	if c.ServiceContent.CustomFieldsManager == nil {
  		return nil, ErrNotSupported
  	}
  	return NewCustomFieldsManager(c), nil
  }
docs: // GetCustomFieldsManager wraps NewCustomFieldsManager, returning ErrNotSupported
  // when the client is not connected to a vCenter instance.