abort loading if promises are not supported in sframe

www/common/sframe-boot2.js

@@ -43,5 +43,12 @@ define([
         throw e;
     };
 
+    if (typeof(Promise) !== 'function') {
+        return void setTimeout(function () {
+            var s = "Internet Explorer is not supported anymore, including by Microsoft.\n\nMost of CryptPad's collaborative functionality requires a modern browser to work.\n\nWe recommend Mozilla Firefox.";
+            window.alert(s);
+        });
+    }
+
     require([document.querySelector('script[data-bootload]').getAttribute('data-bootload')]);
 });