I18N datetime utility file

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

This commit only targets Vanilla JS files.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

app/assets/javascripts/lib/utils/datetime_utility.js

@@ -81,7 +81,7 @@ export const getDayName = date =>
  */
 export const formatDate = datetime => {
   if (_.isString(datetime) && datetime.match(/\d+-\d+\d+ /)) {
-    throw new Error('Invalid date');
+    throw new Error(__('Invalid date'));
   }
   return dateFormat(datetime, 'mmm d, yyyy h:MMtt Z');
 };

locale/gitlab.pot

@@ -5384,6 +5384,9 @@ msgstr ""
 msgid "Invalid Login or password"
 msgstr ""
+msgid "Invalid date"
+msgstr ""
+
 msgid "Invalid feature"
 msgstr ""