Clean up environment if creation is cancelled midway.

[karthiknadig]

Currently we use withProgress API to track the actual environment creation progress. But the cancellation from the notification should be handled better.

1. Partial environment creation must be cleaned up for both venv and conda.
2. CancellationError must be handled and exposed via command.

[eleanorjboyd]

no longer relevant as this all goes through the environment extension and that has a very different logic / cancelation process