https://lib.rs/crates/anyhow

Gives you an Error trait for easy idiomatic error handling.

use anyhow::*;
use std::{fs, io};

const ACPI_PATH: &str = "/sys/class/backlight/";

fn main() -> Result<()> {
	let mut entries = fs::read_dir(ACPI_PATH)?
		.map(|res| res.map(|entry| entry.path()))
		.collect::<Result<Vec<_>, io::Error>>()?;

	// Exit if no backlights found.
	if entries.is_empty() {
		return Err(anyhow!("No backlights found."));
	}

	// Get brightness and max brightness.
	entries.sort();
	dbg!(entries);
	Ok(())
}