I18N of JS files starting with g

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/gl_field_error.js

 import $ from 'jquery';
+import { __ } from '~/locale';
 
 /**
  * This class overrides the browser's validation error bubbles, displaying custom
@@ -61,7 +62,7 @@ export default class GlFieldError {
     this.inputElement = $(input);
     this.inputDomElement = this.inputElement.get(0);
     this.form = formErrors;
-    this.errorMessage = this.inputElement.attr('title') || 'This field is required.';
+    this.errorMessage = this.inputElement.attr('title') || __('This field is required.');
     this.fieldErrorElement = $(`<p class='${errorMessageClass} hidden'>${this.errorMessage}</p>`);
 
     this.state = {

app/assets/javascripts/gpg_badges.js

@@ -20,7 +20,7 @@ export default class GpgBadges {
     const endpoint = tag.data('signaturesPath');
     if (!endpoint) {
       displayError();
-      return Promise.reject(new Error('Missing commit signatures endpoint!'));
+      return Promise.reject(new Error(__('Missing commit signatures endpoint!')));
     }
 
     const params = parseQueryStringIntoObject(tag.serialize());

app/assets/javascripts/groups_select.js

@@ -2,6 +2,7 @@ import $ from 'jquery';
 import axios from './lib/utils/axios_utils';
 import Api from './api';
 import { normalizeHeaders } from './lib/utils/common_utils';
+import { __ } from '~/locale';
 
 export default function groupsSelect() {
   import(/* webpackChunkName: 'select2' */ 'select2/select2')
@@ -18,7 +19,7 @@ export default function groupsSelect() {
           : Api.groupsPath;
 
         $select.select2({
-          placeholder: 'Search for a group',
+          placeholder: __('Search for a group'),
           allowClear: $select.hasClass('allowClear'),
           multiple: $select.hasClass('multiselect'),
           minimumInputLength: 0,

locale/gitlab.pot

@@ -6058,6 +6058,9 @@ msgstr ""
 msgid "Mirroring settings were successfully updated."
 msgstr ""
 
+msgid "Missing commit signatures endpoint!"
+msgstr ""
+
 msgid "MissingSSHKeyWarningLink|add an SSH key"
 msgstr ""
 
@@ -8376,6 +8379,9 @@ msgstr ""
 msgid "Search files"
 msgstr ""
 
+msgid "Search for a group"
+msgstr ""
+
 msgid "Search for projects, issues, etc."
 msgstr ""