server: add exceed_context_size_error type

[ngxson]

Prepare for #14839

Added a new error type exceed_context_size_error

When a request exceed slot context size, error response will now include the token count:

{
    "error": {
        "code": 500,
        "message": "the request exceeds the available context size. try increasing the context size or enable context shift",
        "type": "exceed_context_size_error",
        "n_prompt_tokens": 1407,
        "n_ctx": 256
    }
}