custom_validation_error_message.php

<?php
/**
 * adds a custom validation error message
 *
 * @param array $error_messages
 * @return array
 */
function xnau_add_url_validation_error_message( $error_messages )
{
  // add our message with a placeholder for the field name
  $error_messages['invalid url'] = 'You must provide a valid URL for the %s field.';
  return $error_messages;
}
add_filter( 'pdb-validation_error_messages', 'xnau_add_url_validation_error_message' );