Webform default validation message altering
For Drupal 8, Webform uses Drupal's default Email Form Element. You'll have to override this element info and add your own validation message there. 

See https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21theme.api.php/function/hook_element_info_alter/8.5.x