Better way to notify the user to install pysen-ls

[sergeant-wizard]

Hello I like the project.

I understand most users will read the README and install pysen-ls accordingly, but when the user doesn't,
it takes a while for VSCode to show the user these two pop-up windows.

The user also needs to navigate to the "output" window to see that pysen-ls is not found.

I think it'll improve the user experience if the error windowed showed immediately and notified the user that pysen-ls is not found in the corresponding environment.

[bonprosoft]

Sounds good 👍
It is better to show an error window that says "pysen-ls is unavailable" in such an environment.