LPS-50030 Deleting the last message of a thread sets the last post to its parent message
LPS-50030 match prevAndNextThreads var in this same class
LPS-48728 make DLFileShortcutTrashHandler tests pass

Two kinds of exceptions where thrown here.  Always throw
InvalidRepositoryIdException so that callers don't have to double check.
LPS-48728 make tests expect the correct exn
LPS-48728 SF