Fix a couple of todos.

src/main.rs

@@ -18,6 +18,11 @@ use pbr::{ProgressBar, Units};
 
 mod bbp;
 
+fn print_usage(program: &str, opts: Options) {
+    let brief = format!("Usage: {} [option] INPUT OUTPUT", program);
+    print!("{}", opts.usage(&brief));
+}
+
 fn main() {
     let args: Vec<String> = env::args().collect();
     let program = args[0].clone();
@@ -29,14 +34,28 @@ fn main() {
     opts.optflag("h", "help", "display this help and exit");
 
     let matches = match opts.parse(&args[1..]) {
-        Ok(m) => { m }
-        Err(f) => { panic!(f.to_string()) }
+        Ok(m) => m,
+        Err(_) => {
+            print_usage(&program, opts);
+            std::process::exit(1);
+        }
     };
 
     if matches.opt_present("help") {
-        let brief = format!("Usage: {} [option] INPUT OUTPUT", program);
-        print!("{}", opts.usage(&brief));
-        return;
+        print_usage(&program, opts);
+        std::process::exit(0);
+    }
+
+    if matches.free.len() == 1 {
+        println!("Missing operand: OUTPUT");
+        println!("Try `{} --help` for more information.", program);
+        std::process::exit(1);
+    }
+
+    if matches.free.len() == 0 {
+        println!("Missing operand: INPUT OUTPUT");
+        println!("Try `{} --help` for more information.", program);
+        std::process::exit(1);
     }
 
     if matches.opt_present("c") {
@@ -44,8 +63,7 @@ fn main() {
         let output = &matches.free[1];
 
         compress(&input, &output);
-    }
-    else if matches.opt_present("d") {
+    } else if matches.opt_present("d") {
         let input = &matches.free[0];
         let output = &matches.free[1];