how to return error as a Json from FromParam about rocket HOT 3 CLOSED

use rocket::{launch};

use rocket::{get, routes};
use rocket::http::{ContentType, Status};
use rocket::local::blocking::Client;
use rocket::request::FromParam;
use rocket::response::status::{Custom};
use rocket::serde::json::{Json, json};
use rocket::serde::uuid::Uuid;
use serde_derive::Serialize;

struct CustomerId(Uuid);

#[derive(Debug, Serialize)]
struct ErrorApiOutput {
    error: String,
}

impl<'r> FromParam<'r> for CustomerId {
    type Error = Custom<Json<ErrorApiOutput>>;

    fn from_param(param: &'r str) -> Result<Self, Self::Error> {
        match Uuid::parse_str(param) {
            Ok(uuid) => Ok(CustomerId(uuid)),
            Err(_) => Err(Custom(Status::UnprocessableEntity, Json(ErrorApiOutput { error: "malformed uuid".to_string() }))),
        }
    }
}

type ApiResult<T> = Result<T, Custom<Json<ErrorApiOutput>>>;

#[get("/customers/<customer_id>")]
pub fn get_customer(customer_id: Result<CustomerId, Custom<Json<ErrorApiOutput>>>) -> Result<String, Custom<Json<ErrorApiOutput>>> {
    let id = customer_id?;
    Ok(id.0.to_string())
}

#[launch]
fn rocket() -> _ {
    rocket::build().mount("/api", routes![get_customer])
}

#[test] // working
fn test_get_customer_correct_uuid() {
    let client = Client::tracked(rocket()).expect("valid Rocket instance");

    let response = client.get("/api/customers/550e8400-e29b-41d4-a716-446655440000").dispatch();

    assert_eq!(response.status(), Status::Ok);
    assert_eq!(
        response.into_string(),
        Some("550e8400-e29b-41d4-a716-446655440000".to_string())
    );
}

#[test] // working
fn test_get_customer_incorrect_uuid() {
    let client = Client::tracked(rocket()).expect("valid Rocket instance");

    let response = client.get("/api/customers/invalid-uuid").dispatch();

    assert_eq!(response.status(), Status::UnprocessableEntity);
    assert_eq!(response.content_type(), Some(ContentType::JSON));
    assert_eq!(
        response.into_string(),
        Some(json!({"error": "malformed uuid"}).to_string())
    );
}