Avoid panic for missing selector

Author: kachick

scraper/src/main.rs

@@ -119,7 +119,10 @@ fn main() {
         Output::Html
     };
 
-    let selector = matches.free.first().expect("missing selector");
+    let selector = matches.free.first().unwrap_or_else(|| {
+        eprintln!("missing selector");
+        process::exit(EX_USAGE);
+    });
     let files = &matches.free[1..];
 
     let selector = Selector::parse(selector).unwrap();