I18N EE burndown chart index 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

ee/app/assets/javascripts/burndown_chart/index.js

@@ -4,7 +4,7 @@ import BurndownChart from './burndown_chart';
 import BurndownChartData from './burndown_chart_data';
 import Flash from '~/flash';
 import axios from '~/lib/utils/axios_utils';
-import { s__ } from '~/locale';
+import { s__, __ } from '~/locale';
 
 export default () => {
   // handle hint dismissal
@@ -71,6 +71,6 @@ export default () => {
         window.addEventListener('resize', () => chart.animateResize(1));
         $(document).on('click', '.js-sidebar-toggle', () => chart.animateResize(2));
       })
-      .catch(() => new Flash('Error loading burndown chart data'));
+      .catch(() => new Flash(__('Error loading burndown chart data')));
   }
 };

locale/gitlab.pot

@@ -5034,6 +5034,9 @@ msgstr ""
 msgid "Error loading branches."
 msgstr ""
 
+msgid "Error loading burndown chart data"
+msgstr ""
+
 msgid "Error loading file viewer."
 msgstr ""