429 errors handling

[yw791772]

Hi, I'm following the astream method in the langserve client code, and I have a try catch around the async sse code.
For testing purpose, I use an api key that has no quota, and I can see the 429 errors from the langserve app logs.
But when I looking at the decoded message from it on the client side, I only see 500 with internal server error.

Any guidance on how I can surface the 429 errors on the client side, so I can build specific around this error

[eyurtsev]

Opened an issue for this: #481

Fixing this behavior will also help likely: #414

For now, feel free to fork the code and updating the lines here:

langserve/langserve/api_handler.py

Line 1214 in e1cec83

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