fix(variable_assigner): change VariableOperatorNodeError to inherit from ValueError (#11951)

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/core/workflow/nodes/variable_assigner/common/exc.py

@@ -1,4 +1,4 @@
-class VariableOperatorNodeError(Exception):
+class VariableOperatorNodeError(ValueError):
     """Base error type, don't use directly."""
 
     pass