Question: how to implement custom error handler for 404 not found

[CSRaghunandan]

I want to send JSON responses for all types of errors.

I tried to do something like for 404 not found errors, but not sure how to proceed further:

pub fn not_found_error_handler<B>(
    mut res: dev::ServiceResponse<B>,
) -> actix_web::Result<middleware::ErrorHandlerResponse<B>> {
    let req = res.request();
    let res = res.map_body(|_, _| {/* something needs to go here to modify response body to include json */});
    Ok(middleware::ErrorHandlerResponse::Response(res))
}

I'm using the latest actix web beta version 18