Improved nnUnet Results_Folder validation
Summary: Results_Folder is where pre-trained models are kept. This diff adds better validation by checking if certain critical files exist where expected. Ref: T29238
Test Plan: NA
Reviewers: kislinsk, O1 MITK Reviewer Group I, floca
Reviewed By: kislinsk, O1 MITK Reviewer Group I, floca
Subscribers: floca, kalali
Differential Revision: https://phabricator.mitk.org/D693