Update de.js

The german errorLoading message was missing.

This closes https://github.com/select2/select2/pull/4711.
This closes https://github.com/select2/select2/pull/4591.

src/js/select2/i18n/de.js

@@ -1,6 +1,9 @@
 define(function () {
   // German
   return {
+    errorLoading: function () {
+      return 'Die Ergebnisse konnten nicht geladen werden.';
+    },
     inputTooLong: function (args) {
       var overChars = args.input.length - args.maximum;