restore "unsaved changes" warning on unload

Autosave is triggered at this event, but it seems to fail sometimes (perhaps when it's too slow?).

closes #3475

IPython/html/static/notebook/js/notebook.js

@@ -345,9 +345,13 @@ var IPython = (function (IPython) {
             if (kill_kernel) {
                 that.kernel.kill();
             }
-            // if we are autosaving, trigger an autosave on nav-away
-            if (that.dirty && that.autosave_interval && ! that.read_only) {
-                that.save_notebook();
+            // if we are autosaving, trigger an autosave on nav-away.
+            // still warn, because if we don't the autosave may fail.
+            if (that.dirty && ! that.read_only) {
+                if ( that.autosave_interval ) {
+                    setTimeout(function() { that.save_notebook(); }, 0);
+                }
+                return "Unsaved changes will be lost.";
             };
             // Null is the *only* return value that will make the browser not
             // pop up the "don't leave" dialog.