Show a warning if there is no kernel installed

[martinRenou]

If there is no kernel installed, the extension fails with:

   File "/home/runner/micromamba/envs/build-env/lib/python3.12/pathlib.py", line 1056, in iterdir
    for name in os.listdir(self):
                ^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmp2ay0_eyo/env/envs/xeus-env/share/jupyter/kernels'

It would be nice to have a nice user-friendly message instead.

[martinRenou]

Fixed in #37