Status code of HTTP error messages is not 4xx or 5xx

[lucasavila00]

I can fix that. It's similar to #80

Here: https://github.com/EricLBuehler/mistral.rs/blob/master/mistralrs-server/src/main.rs#L140

e.to_string().into_response() does not carry the information that string is of an error, thus we get wrong HTTP status code

In the end this makes the OpenAI client return an unhelpful message:

Traceback (most recent call last):
  File "/home/lucas/oss/mistral.rs/examples/server/chat.py", line 25, in <module>
    resp = completion.choices[0].message.content
AttributeError: 'str' object has no attribute 'choices'

[EricLBuehler]

Thanks for raising this! If you can submit a PR that would be great.