Show error notification when cell drag fails

[andrii-i]

Problem

When dragging notebook cells fails (due to missing notebook tracker, cell not found, or other issues), show a user-visible error notification instead of only logging error to console (which a lot of users might not see / check).

Proposed solution

May also be worth opening a Notification.error() popup for visibility.

Originally posted by @dlqqq in #248 (comment)