Throw proper exception with message in MenuBase64Converter

Shoko.Server/Utilities/MediaInfoLib/MenuBase64Converter.cs

@@ -38,7 +38,7 @@ public override object ReadJson(JsonReader reader, Type objectType, object exist
                         break;
                     }
                     default:
-                        throw new NotImplementedException();
+                        throw new NotSupportedException("Unknown type encountered in MenuStream.extra");
                 }
             }