If extraction fails at part 19, use WinRAR or 7-Zip to "Test" the archive. You may need to redownload just this part if it's corrupted.
If you are having trouble with this specific part or the overall installation: Forza.Horizon.5-CODEX.part19.rar
Check out how to find the absolute rarest cars in the game once you get it running: If extraction fails at part 19, use WinRAR