Update modal.py (#11588)

feat: Raise KeyError when 'prompt' key is missing in JSON response

This commit updates the error handling in the code to raise a KeyError
when the 'prompt' key is not found in the JSON response. This change
makes the code more explicit about the nature of the error, helping to
improve clarity and debugging.

@baskaryan, @eyurtsev.

libs/langchain/langchain/llms/modal.py

@@ -91,7 +91,7 @@ class Modal(LLM):
             if prompt in response.json()["prompt"]:
                 response_json = response.json()
         except KeyError:
-            raise ValueError("LangChain requires 'prompt' key in response.")
+            raise KeyError("LangChain requires 'prompt' key in response.")
         text = response_json["prompt"]
         if stop is not None:
             # I believe this is required since the stop tokens