Avoid panic for invalid selector

Author: kachick

scraper/src/main.rs

@@ -125,7 +125,10 @@ fn main() {
     });
     let files = &matches.free[1..];
 
-    let selector = Selector::parse(selector).unwrap();
+    let selector = Selector::parse(selector).unwrap_or_else(|e| {
+        eprintln!("failed to parse selector: {}", e);
+        process::exit(EX_USAGE);
+    });
 
     let matched = if files.is_empty() {
         query(&input, &output, &selector, &mut io::stdin())