Warn user when an invalid `dart.flutterSdkPath` is provided

[felangel]

Is your feature request related to a problem? Please describe.
As a developer, I tried to override the "dart.flutterSdkPath" in my settings.json and was setting it to ~Development/github.com/flutter/flutter/bin/flutter but the extension expects: ~Development/github.com/flutter/flutter.

Describe the solution you'd like
I would expect an error dialog to alert me when my "dart.flutterSdkPath" is invalid and therefore ignored.

Additional context
Related to #4607