display an error in the executable fields if the specified executable isn't found

[DetachHead]

Feature or enhancement

currently there doesn't seem to be any visible error when the specified pyright executables are not found:

Proposed solution

maybe the text can turn red like in the typescript settings:

Alternatives considered

No response

Additional context

No response

[InSyncWithFoo]

There used to be such a feature prior to #79. These were what I thought back then:

• The user would need to input the path manually (terrible UX, I know), and so there needed to be some kind of safeguard. The big rewrite added executable detection, making it far less necessary.
• I don't want to prevent the user from saving their settings just because some value does not appear to be right, since that value could theoretically come from anywhere (e.g., shared via VCS).

I'll see if I can implement a warning-only validation.