Clear the saved jupyter interpreter path if it fails to launch

[IanMatthewHuff]

1. Launch jupyter from an interpreter (you want it to be cached in the jupyter interpreter memento).
2. Now remove that interpreter from your system
3. Open a new workspace and point VSCode at a valid jupyter installed interpreter
4. Try to execute a cell

Result:
Jupyter will not start. However you won't get a popup on this or a notification of what interpreter it was trying to use. You just get a call stack of our code (NativeEditor.startServer ect..) displayed in the output cell.

Expected:
Any of the following would help:

1. Show the interpreter that we are trying to use, so I could at least tell what what happening
2. Guide the user to the "Select Jupyter Interpreter" command for a failing cached interpreter
3. Clear the saved interpreter if it fails so we move back to using the VSCode selected interpreter.

[rchiodo]

I vote for number 3

[rchiodo]

This feels like a regression to me. It wouldn't have done this before

[IanMatthewHuff]

Yeah, folks won't be removing interpreters all the time, but it's not that rare. And it's pretty bad when you get into this state. Plus the user has no idea that the first time they successfully launch Jupyter that they have set this internal setting.

[IanMatthewHuff]

Note. I was not able to repro the situation where I was failing and didn't see the missing dependencies popup. When I deleted or removed jupyter from the saved jupyter interpreter I always got the missing dependencies popup. So I added the target interpreter to that dialog (so the user can see what they are trying to use) and improved the help link.

[rchiodo]

Hmm. I can repro this if I set the jupyter interpreter to something that doesn't support it. Like a virtual env with jupyter.

[IanMatthewHuff]

@rchiodo Sorry what's an example of this "Like a virtual env with jupyter." A virtual env with Jupyter installed should work just fine. Do you mean if you delete that env?

[rchiodo]

On windows I can't get a virtualenv (one created with venv) to work. I installed the default python from the store, then create a venv for juptyer, pip install jupyter and try running a notebook. The kernel will crash.

If you set this as your selected jupyter interpreter, we never recover.

[IanMatthewHuff]

Hmm virtualenvs from venv work just fine for me. Not fully sure if I really want to fudge up my system by installing the default python. I'll try to check it out this afternoon.

[DonJayamanne]

Works for me.
Then again, I'm on a Mac

[rchiodo]

Validated. Removed jupyter from a selected interpreter that was already working and it just moved onto the new one automatically.

[DonJayamanne]

Validated