Print usage on misusage

src/bin/mesh/main.rs

@@ -1,7 +1,7 @@
 use glam::*;
 use refraction::mesh_loader::load_mesh;
 use refraction::mesh_tracer::{trace_to_mesh, Mesh};
-use show_image::{ImageInfo, ImageView, WindowOptions};
+use show_image::{exit, ImageInfo, ImageView, WindowOptions};
 use std::env;
 use std::error::Error;
 use std::f32::consts::PI;
@@ -91,6 +91,10 @@ fn render(mesh: &Mesh, camera: impl Fn(Vec2) -> (Vec3, Vec3)) -> Image {
 #[show_image::main]
 fn main() -> Result<(), Box<dyn Error>> {
 	let args: Vec<String> = env::args().collect();
+	if args.len() != 2 {
+		println!("Usage: {} path/to/model.obj", args[0]);
+		exit(1);
+	}
 	let mesh = {
 		let f = File::open(&args[1])?;
 		let mut f = BufReader::new(f);