Refactored the workspace.Delete() action into a TryToDeleteWorkspace method.

Includes alternate Retry.Do() values of 5 second delay between tries to help with busy TFS servers.
Merge pull request #657 from JenasysDesign/f-TryToDeleteWorkspaceAgain

Add TryToDeleteWorkspace method for reliability
Retry after getting a file from TFS if the status is not satisfying

Should improve reliability.
Hope it will fix #444
Encapsulate `NumFailures != 0`.