Duplicate tasks when private type index entry is missing or fails to load

[jg10-mastodon-social]

To replicate:

1. Create a new workspace
2. Create a task
3. Separately delete the workspace from the private type index
4. Sync

Focus will create a new task list and duplicate all tasks recursively within the existing task list

I think the expected behaviour should instead be that a sync error is raised that the workspace is not known.

I've seen the error happen because the private type index failed to load (some kind of network error), so it's not always the case that the entry is actually missing from the private type index.