LPS-50058 Follow backup repository name requirements
LPS-50058 we need to do this or else the returned backup name may be different
LPS-50058 more accurate
LPS-50058 more accurate
LPS-50058 Sort
LPS-50058 SF
LPS-50058 consistency
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