[Segmentation] Fix several crashes in group deletion edge cases

Summary:

• Consider possibility that no label is selected when trying to delete a group
• Make messages for group deletion more explicit and clear
• When searching for a closest label instance as candidate for next selection, consider all label instances on a level, before moving up the tree hierarchy

Fixes T30252

Test Plan: - Follow instructions in T30252

Reviewers: O1 MITK Reviewer Group I, s434n, floca

Reviewed By: O1 MITK Reviewer Group I, s434n, floca

Subscribers: a178n, floca, s434n

Maniphest Tasks: T30252

Differential Revision: https://phabricator.mitk.org/D884