Handle Result in doctest hmadc
//! let msg = "Some message.";
//!
//! let mut mac = hmac::init(&key);
-//! mac.update(msg.as_bytes());
+//! mac.update(msg.as_bytes()).unwrap();
//! mac.finalize().unwrap();
//! ```
//! ### Verifying HMAC:
//! let msg = "Some message.";
//!
//! let mut mac = hmac::init(&key);
-//! mac.update(msg.as_bytes());
+//! mac.update(msg.as_bytes()).unwrap();
//!
//! assert!(hmac::verify(&mac.finalize().unwrap(), &key, msg.as_bytes()).unwrap());
//! ```
