Improve error message when trying to debug with no interpreter is selected

[luabud]

Testing #36

To reproduce, open a Python project, select the automatic config to run a Python file from debugpy. Then select an invalid interpreter (e.g. run the "Select Interpreter" command, click on "enter interpreter path", add something that doesn't exist like "bla", hit enter and ignore the notification that shows up).
When you start debugging with the debugpy extension, this error message shows up:

For contrast, this is what shows up when debugging a the Python extension config:

I actually like the modal window because it is a blocking action, but if we could improve the message to something like "Invalid Python interpreter. Please select a different one." And then provide a button with "Select Python Interpreter", that would be ideal IMO 😊