we don't want to panic!

src/main.rs

@@ -125,7 +125,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .get_matches();
 
     let mut robot_pool: Vec<Robot> = Vec::new();
-    let raw_conf = open_file(matches.value_of("file").unwrap())?;
+    let raw_conf = open_file(matches.value_of("file").unwrap_or("two_robots.txt"))?;
     Ok(())
 }