Add missing errorLoading to Turkish translation

This closes https://github.com/select2/select2/pull/4650.

src/js/select2/i18n/tr.js

@@ -1,6 +1,9 @@
 define(function () {
   // Turkish
   return {
+    errorLoading: function (){
+      return "Sonuç yüklenemedi";
+    },
     inputTooLong: function (args) {
       var overChars = args.input.length - args.maximum;