Improve deno jupyter installation DX

[littledivy]

• [ ] Improve error message when jupyter not found or kernelspec is not installed. Current error message is confusing:

  
  $ deno jupyter --unstable
  error: Failed to get list of installed kernelspecs

Caused by: program not found

- [ ] False positive kernel installation when `jupyter` command not in Path. 

To reproduce:

```shell
# Assumes jupyter is not installed already

$ deno jupyter --unstable --install
✅ Deno kernelspec installed successfully.

$ pip install jupyterlab
$ python3 -m jupyterlab
# !! Deno kernel is not listed

[tiagofassoni]

Thank you! I didn't understand what was happening to the jupyter kernel installation and your issue told me what I needed to do.

[Martin978]

• [ ] Mejore el mensaje de error cuando no se encuentra jupyter o kernelspec no está instalado. El mensaje de error actual es confuso:

$ deno jupyter --unstable
error: Failed to get list of installed kernelspecs

Caused by:
    program not found

• [ ] Instalación del kernel falso positivo cuando jupyterel comando no está en la ruta.

Reproducir:

# Assumes jupyter is not installed already

$ deno jupyter --unstable --install
✅ Deno kernelspec installed successfully.

$ pip install jupyterlab
$ python3 -m jupyterlab
# !! Deno kernel is not listed

Ofertas,subasta