feat: return streaming errors as an event formatted for openai's client

[drbh]

This PR adds the functionality to emit errors as formatted events.

Note the changes look large because of a new indent however the main logic that changed is the addition of

Err(err) => {
    let error_event: ErrorEvent = err.into();
    let event = Event::default().json_data(error_event).unwrap_or_else(|e| {
        InferError::StreamSerializationError(e.to_string()).into()
    });
    yield Ok::<Event, Infallible>(event);
    break;
}

this change enables the openai client to pull the error message from the stream and correctly surface it to the user

example script (throws with change included)

import openai
from openai import OpenAI

client = OpenAI(
    api_key="key",
)

client.base_url = "http://127.0.0.1:3000/v1"

try:
    completion = client.chat.completions.create(
        messages=[
            {
                "role": "user",
                "content": "test " * 32_000,
            }
        ],
        model="tgi",
        stream=True,
    )

    for index, message in enumerate(completion):
        print(f"Message {index}: {message}")
except openai.APIError as e:
    print(e)