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