fix minor bugs in export as (nbconvert) example

examples/Interactive Widgets/Export As (nbconvert).ipynb

@@ -20,6 +20,8 b''
       "from IPython.nbconvert import get_export_names, export_by_name\n",
       "from IPython.nbconvert.writers import FilesWriter\n",
       "from IPython.nbformat import current"
+      "from IPython.nbconvert.utils.exceptions import ConversionException\n",
+
      ],
      "language": "python",
      "metadata": {},
@@ -151,7 +153,7 b''
       "        download_link.value = \"<br>Results: <a href='files/{filename}'><i>\\\"{filename}\\\"</i></a>\".format(filename=write_results)\n",
       "        download_link.visible = True\n",
       "        \n",
-      "def handle_export():\n",
+      "def handle_export(widget):\n",
       "    with open(filename, 'r') as f:\n",
       "        export(filename, current.read(f, 'json'))\n",
       "export_button.on_click(handle_export)"