Strip HTML tags from validation errors
hook_services_request_postprocess_alter() looks like a good candidate. I don't have an installation to test with right now but something like this ought to work:

function MYMODULE_services_request_postprocess_alter($controller, $args, &amp;$result) {
  if (isset($result['form_errors'])) {
    array_walk($result['form_errors'], function(&amp;$error) {
      $error = strip_tags($error);
    });
  }
}