Python Language Server "unable to determine path to interpreter" should be actionable

[qubitron]

Environment data

• VS Code version: 1.28.2 (user setup)
• Extension version (available under the Extensions sidebar): 2018.9.1
• OS and version: Windows 10
• Python version (& distribution if applicable, e.g. Anaconda): Have python 2.7, 3.6, 3.7 and anaconda installed
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): None (yet)
• Relevant/affected Python packages and their versions: N/A

Actual behavior

Opened a folder with a single empty hello.py file:

No python interpreter is selected, and I get this error message immediately after opening the file

Expected behavior

The dialog should instead say something like

No Python interpreter is selected, you need to select a Python interpreter to enable features such as IntelliSense, linting, and debugging.
[ Select Python Interpreter ] 

[d3r3kk]

Current behaviour is indeed unactionable:

I propose we do something like so:

Branch for person who would implement such a thing.

Note: The popup for dependencies such as linters should probably be deferred until after this action is taken as well. Covered in #2811.

[DonJayamanne]

@d3r3kk
I'd change this as follows:

• Modify pythonInterpreter.ts (but only check currently selected interpreter) to add a new diagnostic InvalidPythonInterpreterDiagnostic (that would check if the selected interpreter is valid or not)
• Should be a very simple change.

Benefits:

• Consistent approach to validations/checks
• Consistent UX (displaying message and prompt, e.g. button would read Select Python Interpreter)
• Catch the error early on
• Benefits the entire extension (currently this validation is done in language server code, but linters, formatters, etc all fail due to the invalid python interpreter having been selected).

[DonJayamanne]

Changing label to Needs PR, as that's whats been requested (a change, and the UI has been provided).

[DonJayamanne]

@brettcannon
This is a very simple change that improves the getting started experience, hence changing the label to P1.
As discussed, if you disagree please feel free to change the priority.