Make the language in unconfigured notice more natural

TODO set these automatically when Custom error is enabled if still set to default.

src/Form/CustomerrorSettingsForm.php

@@ -80,7 +80,7 @@ class CustomerrorSettingsForm extends ConfigFormBase {
     ];
     foreach ($errors as $code => $desc) {
       if (\Drupal::config('system.site')->get("page.$code") != "/customerror/{$code}") {
-        drupal_set_message($this->t('Error reporting is not set for error @error. Please ensure that the default @error page is set to be /customerror/@error on the @link.', ['@error' => $code, '@link' => Link::createFromRoute($this->t('Site information settings page'), 'system.site_information_settings')->toString()]), 'error', FALSE);
+        drupal_set_message($this->t('Custom error is not configured for @error errors. Please ensure that the default @error page is set to be /customerror/@error on the @link.', ['@error' => $code, '@link' => Link::createFromRoute($this->t('Site information settings page'), 'system.site_information_settings')->toString()]), 'error', FALSE);
       }
     }
     foreach ($errors as $code => $desc) {