persist_file() can return a Deferred that is never awaited

[wRAR]

All non-local FilesPipeline store classes (S3FilesStore, GCSFilesStore, FTPFilesStore) return a Deferred from the persist_file() method but FilesPipeline.file_downloaded() ignores the result of this method. Looks like it was this way since the beginning.

I don't have suggestion for a fix because file_downloaded is a sync method.