T30392 Totalsegmentator & Otsu crash with multi label refactor

Summary:
Fixed crash when labels are deleted

Make message boxes more informative

• Little code clean up - Remove unnecessary suffix in otsu label names

Test Plan: code review, try to reproduce error of T30392

Reviewers: O1 MITK Reviewer Group I, a178n, kislinsk

Reviewed By: O1 MITK Reviewer Group I, a178n, kislinsk

Subscribers: kislinsk, s434n, a178n

Differential Revision: https://phabricator.mitk.org/D924