I18N boards store directory

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/boards/stores/actions.js

+import { __ } from '~/locale';
+
 const notImplemented = () => {
-  throw new Error('Not implemented!');
+  throw new Error(__('Not implemented!'));
 };
 
 export default {

app/assets/javascripts/boards/stores/mutations.js

 import * as mutationTypes from './mutation_types';
+import { __ } from '~/locale';
 
 const notImplemented = () => {
-  throw new Error('Not implemented!');
+  throw new Error(__('Not implemented!'));
 };
 
 export default {

locale/gitlab.pot

@@ -6470,6 +6470,9 @@ msgstr ""
 msgid "Not found."
 msgstr ""
 
+msgid "Not implemented!"
+msgstr ""
+
 msgid "Not now"
 msgstr ""