Update bageldb.py (#11421)

I have restructured the code to ensure uniform handling of ImportError.
In place of previously used ValueError, I've adopted the standard
practice of raising ImportError with explanatory messages. This
modification enhances code readability and clarifies that any problems
stem from module importation.

libs/langchain/langchain/vectorstores/bageldb.py

@@ -108,7 +108,7 @@ class Bagel(VectorStore):
         try:
             import bagel  # noqa: F401
         except ImportError:
-            raise ValueError("Please install bagel `pip install betabageldb`.")
+            raise ImportError("Please install bagel `pip install betabageldb`.")
         return self._cluster.find(
             query_texts=query_texts,
             query_embeddings=query_embeddings,