fix(dataset): 解决操作符查找失败时的错误处理

- 当操作符在任何注册表中都找不到时抛出ImportError异常
- 对无效的注册表内容类型抛出更具体的ImportError异常
- 提供更清晰的错误信息帮助用户诊断问题

runtime/python-executor/datamate/core/dataset.py

@@ -136,6 +136,10 @@ class RayDataset(BasicDataset):
         if registry_content is None:
             from core.base_op import OPERATORS as RELATIVE_OPERATORS
             registry_content = RELATIVE_OPERATORS.modules.get(op_name)
+
+        if registry_content is None:
+            raise ImportError(f"Operator '{op_name}' not found in any registry. Please check if the operator name is correct.")
+
         if isinstance(registry_content, str):
             # registry_content是module的路径
             submodule = importlib.import_module(registry_content)
@@ -148,7 +152,7 @@ class RayDataset(BasicDataset):
             # registry_content是module本身
             res = registry_content
         else:
-            res = None
+            raise ImportError(f"Invalid registry content for operator '{op_name}': expected str or BaseOp subclass, got {type(registry_content)}")
         return res
 
     def _run_single_op(self, operators_cls, init_kwargs, **kwargs):