Use 'fail if exist' strategy on automatic move

Prevent existing files overwriting when torrent is moved automatically (e.g. when moved from incomplete to final save path).

PR #17855.

src/base/bittorrent/torrentimpl.h

@@ -63,6 +63,7 @@ namespace BitTorrent
 
     enum class MoveStorageMode
     {
+        FailIfExist,
         KeepExistingFiles,
         Overwrite
     };