server: print actual model name in 'model not found" error

Experimenting with AI, my environment gets messy fast and it's not
always easy to know what model my software is trying to load. This helps
with troubleshooting.

before:

Error: {
  code = 400,
  message = "model not found",
  type = "invalid_request_error"
}

After:

Error: {
  code = 400,
  message = "model 'toto' not found",
  type = "invalid_request_error"
}

tools/server/server-models.cpp

@@ -767,7 +767,7 @@ static bool router_validate_model(const std::string & name, server_models & mode
     }
     auto meta = models.get_meta(name);
     if (!meta.has_value()) {
-        res_err(res, format_error_response("model not found", ERROR_TYPE_INVALID_REQUEST));
+        res_err(res, format_error_response(string_format("model '%s' not found", name.c_str()), ERROR_TYPE_INVALID_REQUEST));
         return false;
     }
     if (models_autoload) {