Add ability to translate exceptions

[eyurtsev]

Expose exception translation capability to allow users to provide their own logic for translating server side errors into client side errors

langserve/langserve/api_handler.py

Line 1214 in e1cec83

# the message may contain sensitive information.

Discussed in #473

^{Originally posted by yw791772 February 21, 2024}
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

[yw791772]

2 of the most common use cases I encountered:

• 429 rate limiting
• context too large, which could turn out to be hard to detect, for example: all I see is a 400 bad request from open ai

[eyurtsev]

#545

[philbo]

@eyurtsev do you have any ideas about how to approach implementing this? I can try to contribute a solution