fix: openai errors are correctly proxied

[adubovik]

OpenAI errors received from the upstream are wrapped into a dictionary with detail field instead of being proxied as
is.

It happens because the OpenAI error are wrapped into fastapi.HTTPException, which has necessary this detail field.

The solution was to migrate the error to a more permitting fastapi.JSONResponse.

Along the way, the adapter was migrated to HTTPException from the DIAL SDK.

Error before

{
  "detail": "{\"error\":{\"message\":\"Invalid value: 'users'. Supported values are: 'system', 'assistant', 'user', 'function', and 'tool'.\",\"type\":\"invalid_request_error\",\"param\":\"messages[0].role\",\"code\":\"invalid_value\"}}"
}

Error after

{
  "error": {
    "message": "Invalid value: 'users'. Supported values are: 'system', 'assistant', 'user', 'function', and 'tool'.",
    "type": "invalid_request_error",
    "param": "messages[0].role",
    "code": "invalid_value"
  }
}

[adubovik]

Review discussions

Re aidial_adapter_dial.utils.sse_stream.to_openai_sse_stream:

Q: Before this PR you returned END_CHUNK only in case there is no error. Now it's returned even after exception handling. Is this intentional?
A: we always return CHUNK_CHUNK in DIAL SDK, so I reasoned that we should follow the same logic everywhere

Q: If it's intentional, why you didn't use else/finally there?
A: I don't think we need to use finally here. Imagine SystemExit, CancelError or KeyboarInterrupt exception is thrown (none of which is Exception), the error handler won't catch it, but we will send END_CHUNK. This is strange. I would rather interrupt the response abruptly than make a false impression that everything is ok.