nickserv: IDENTIFY reports an unregistered account distinctly

A failed IDENTIFY now says whether the account isn't registered or the
password was wrong, instead of one message for both.

modules/nickserv/nickserv.rs

@@ -50,6 +50,12 @@ impl Service for NickServ {
                         return;
                     }
                 };
+                // Distinguish an unregistered account from a wrong password, so a
+                // failed login says which it was.
+                if !db.exists(account_name) {
+                    ctx.notice(me, from.uid, format!("\x02{account_name}\x02 isn't registered."));
+                    return;
+                }
                 match db.authenticate(account_name, password) {
                     Some(account) => {
                         // Already identified to this account: don't re-fire the login.

src/engine/mod.rs

@@ -995,6 +995,12 @@ mod tests {
         });
         assert!(bad.iter().any(|a| matches!(a, NetAction::Notice { text, .. } if text.contains("Invalid password"))), "{bad:?}");
         assert!(!bad.iter().any(|a| matches!(a, NetAction::Metadata { .. })), "wrong password must not log in: {bad:?}");
+
+        // An unregistered account says so, rather than "Invalid password".
+        let missing = e.handle(NetEvent::Privmsg {
+            from: "000AAAAAC".into(), to: "42SAAAAAA".into(), text: "IDENTIFY ghost whatever".into(),
+        });
+        assert!(missing.iter().any(|a| matches!(a, NetAction::Notice { text, .. } if text.contains("isn't registered"))), "{missing:?}");
     }
 
     // ChanServ: registration needs identification, INFO shows the founder, and