Don't let extension activation fail, but instead set a flag that's checked before every command invocation

[brettfo]

Currently if there is an issue with the user's dotnet SDK, extension activation fails and no commands get registered, so if the user then tries to create a new notebook, etc., they're given the very unhelpful message that the command isn't defined. If instead we effectively wrap the entire activation in try/catch, we can then set an extension-wide global indicating the status of activation and check this on all command invocations.