fix enum error message (#8652)

could be a string so don't directly call value

libs/langchain/langchain/smith/evaluation/runner_utils.py

@@ -7,6 +7,7 @@ import functools
 import itertools
 import logging
 from datetime import datetime
+from enum import Enum
 from typing import (
     Any,
     Callable,
@@ -345,9 +346,10 @@ def _setup_evaluation(
         else:
             run_type = "chain"
             if data_type in (DataType.chat, DataType.llm):
+                val = data_type.value if isinstance(data_type, Enum) else data_type
                 raise ValueError(
                     "Cannot evaluate a chain on dataset with "
-                    f"data_type={data_type.value}. "
+                    f"data_type={val}. "
                     "Please specify a dataset with the default 'kv' data type."
                 )
             chain = llm_or_chain_factory()