Fix Custom LLM Agent example (#6429)

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The `CustomOutputParser` needs to throw `OutputParserException` when it
fails to parse the response from the agent, so that the executor can
[catch it and
retry](be9371ca8f/langchain/agents/agent.py (L767))
when `handle_parsing_errors=True`.

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#### Who can review?

Tag maintainers/contributors who might be interested: @hwchase17

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docs/snippets/modules/agents/how_to/custom_llm_agent.mdx

@@ -22,7 +22,7 @@ from langchain.agents import Tool, AgentExecutor, LLMSingleActionAgent, AgentOut
 from langchain.prompts import StringPromptTemplate
 from langchain import OpenAI, SerpAPIWrapper, LLMChain
 from typing import List, Union
-from langchain.schema import AgentAction, AgentFinish
+from langchain.schema import AgentAction, AgentFinish, OutputParserException
 import re
 ```
 
@@ -135,7 +135,7 @@ class CustomOutputParser(AgentOutputParser):
         regex = r"Action\s*\d*\s*:(.*?)\nAction\s*\d*\s*Input\s*\d*\s*:[\s]*(.*)"
         match = re.search(regex, llm_output, re.DOTALL)
         if not match:
-            raise ValueError(f"Could not parse LLM output: `{llm_output}`")
+            raise OutputParserException(f"Could not parse LLM output: `{llm_output}`")
         action = match.group(1).strip()
         action_input = match.group(2)
         # Return the action and action input