Return specific error messages during /stream and /astream_events

[philbo]

I would like to be able to add specific error messages to the streamed responses so my client can respond appropriately.

Currently it seems that any exception is purposefully hardcoded to return:

"event": "error",
"data": json.dumps(
      {"status_code": 500, "message": "Internal Server Error"}
  ),

Is there any way to change this behaviour?

To give some wider context, say I have a Chain which scrapes a URL provided by the client and then passes that response content to the LLM with some prompt:

fetch_url | prompt | llm | parser

If the URL is invalid, or returns an error response, I want to stop the chain and return a useful message to the client. I can currently do this with the /invoke endpoint by raising different types of HTTPException errors in my chain.

[eyurtsev]

Here's an issue for this: #481

Duplicate of #473