[extractor/unsupported] Raise error on known DRM-only sites (#5483)

Authored by: coletdjnz

yt_dlp/extractor/_extractors.py

@@ -2023,6 +2023,7 @@
 from .unistra import UnistraIE
 from .unity import UnityIE
 from .unscripted import UnscriptedNewsVideoIE
+from .unsupported import KnownDRMIE
 from .uol import UOLIE
 from .uplynk import (
     UplynkIE,