Interpret as error using unknown fields in variants.

kibot/var_base.py

@@ -10,6 +10,7 @@ from .macros import macros, document  # noqa: F401
 class BaseVariant(RegVariant):
     def __init__(self):
         super().__init__()
+        self._unkown_is_error = True
         with document:
             self.name = ''
             """ Used to identify this particular variant definition """