Change "invalid interpreter" notification to a modal dialog

[luabud]

Right now when there's an invalid interpreter selected for the workspace and you try to run a file or start the REPL, for example, we display an error notification. Feedback from the broader team was that we should probably change it to a modal dialog since it's an "blocking" error, to drive higher discoverability of the failure point.

[luabud]

I think this is an example of a modal dialog in the Jupyter extension: https://github.com/microsoft/vscode-jupyter/blob/main/src/notebooks/controllers/commands/installPythonControllerCommands.ts#L52