Kernel not available not captured -> Error 500

[wolfv]

I just accidentally pressed the voila button on a Python 2 notebook and don't have a Python 2 Kernel installed. That resulted in a ugly 500.
Maybe we can catch the exception and report something nicer?

[E 02:25:44.467 LabApp] Uncaught exception GET /voila/render/examples/notebooks/ROS%203D%20Grid.ipynb (172.17.0.1)
    HTTPServerRequest(protocol='http', host='localhost:1234', method='GET', uri='/voila/render/examples/notebooks/ROS%203D%20Grid.ipynb', version='HTTP/1.1', remote_ip='172.17.0.1')
    Traceback (most recent call last):
      File "/usr/local/lib/python3.5/dist-packages/tornado/web.py", line 1699, in _execute
        result = await result
      File "/usr/lib/python3.5/asyncio/futures.py", line 363, in __iter__
        return self.result()  # May raise too.
      File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
        raise self._exception
      File "/usr/local/lib/python3.5/dist-packages/tornado/gen.py", line 736, in run
        yielded = self.gen.throw(*exc_info)  # type: ignore
      File "/usr/local/lib/python3.5/dist-packages/voila/handler.py", line 56, in get
        kernel_id = yield tornado.gen.maybe_future(self.kernel_manager.start_kernel(kernel_name=kernel_name, path=cwd))
      File "/usr/local/lib/python3.5/dist-packages/tornado/gen.py", line 729, in run
        value = future.result()
      File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
        raise self._exception
      File "/usr/local/lib/python3.5/dist-packages/tornado/gen.py", line 209, in wrapper
        yielded = next(result)
      File "/usr/local/lib/python3.5/dist-packages/notebook/services/kernels/kernelmanager.py", line 160, in start_kernel
        super(MappingKernelManager, self).start_kernel(**kwargs)
      File "/usr/local/lib/python3.5/dist-packages/jupyter_client/multikernelmanager.py", line 110, in start_kernel
        km.start_kernel(**kwargs)
      File "/usr/local/lib/python3.5/dist-packages/jupyter_client/manager.py", line 246, in start_kernel
        kernel_cmd = self.format_kernel_cmd(extra_arguments=extra_arguments)
      File "/usr/local/lib/python3.5/dist-packages/jupyter_client/manager.py", line 170, in format_kernel_cmd
        cmd = self.kernel_spec.argv + extra_arguments
      File "/usr/local/lib/python3.5/dist-packages/jupyter_client/manager.py", line 82, in kernel_spec
        self._kernel_spec = self.kernel_spec_manager.get_kernel_spec(self.kernel_name)
      File "/usr/local/lib/python3.5/dist-packages/jupyter_client/kernelspec.py", line 236, in get_kernel_spec
        raise NoSuchKernel(kernel_name)
    jupyter_client.kernelspec.NoSuchKernel: No such kernel named python2
[E 02:25:44.589 LabApp] {
      "Accept-Language": "en,en-US;q=0.9,de;q=0.8,es;q=0.7,fr;q=0.6",
      "Upgrade-Insecure-Requests": "1",
      "Dnt": "1",
      "Connection": "keep-alive",
      "Accept-Encoding": "gzip, deflate, br",
      "User-Agent": "Mozilla/5.0 (X11; Fedora; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.90 Safari/537.36",
      "Cookie": "_xsrf=2|e3d48a54|9df6b536dd80ab997fdfcb68cfaf1597|1516263768; org.cups.sid=3987dbf3d88f88015acc753740791e03; _ga=GA1.1.375891167.1516910868; host=.butterfly; TWISTED_SESSION=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2luZm8iOnsiYW5vbnltb3VzIjp0cnVlfSwiZXhwIjoxNTI3NzAzNDAxfQ.MLpNPRrGKOREpEx8-tiVDMuDWpYJ_gvLIL1QjzD5LG0; username-localhost-8890=\"2|1:0|10:1559647541|23:username-localhost-8890|44:N2ZmODc1OWQ1YzIxNGI3ZGE0YzdmMzlkOTUzYmIxNjQ=|05c989f0917e21e7d152e977c335ed2c3a43f182514be38b767c086d9d309975\"; username-localhost-8889=\"2|1:0|10:1560354246|23:username-localhost-8889|44:ODBmYTFkOWQ1ZGVkNGU1MGEwYjM2OThiM2NiYjI2MTM=|910b7e34253f71a316be8292c75c9c1110b27783760348fbc29670b58d233ce6\"; username-localhost-8888=\"2|1:0|10:1560417646|23:username-localhost-8888|44:ZTQ5ODAwYjU1YWMxNDAxODg4NGQ3ZTBmZjE0NjE3YTM=|18752bc255147a1b40c54bddf3d7af7fae56f6c4f9bafc6c917c56b4bd83d951\"; mjx.latest=2.7.5",
      "Referer": "http://localhost:1234/lab/workspaces/shsh",
      "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3",
      "Host": "localhost:1234"
    }

[wolfv]

also this wouldn't have happened if the voila button automatically saves the notebook :)

[SylvainCorlay]

Thanks! I think I should close as a duplicate of #259

[wolfv]

Sure

[maartenbreddels]

also this wouldn't have happened if the voila button automatically saves the notebook :)

indeed, totally agree #257

[jtpio]

also this wouldn't have happened if the voila button automatically saves the notebook :)

Will be fixed in #262 :)