langchain/chains/llm_bash/base.py/LLMBashChain/validate_prompt
class LLMBashChain:    def validate_prompt(cls, values: Dict) -> Dict:
        if values["llm_chain"].prompt.output_parser is None:
            raise ValueError(
                "The prompt used by llm_chain is expected to have an output_parser."
            )
        return values