show error on unknown command

lib/api/tag.js

@@ -257,6 +257,8 @@ module.exports = {
             update(host, tags, source, callback);
         } else if(subcommand === "delete"){
             remove(host, source, callback);
+        } else {
+            callback("unknown subcommand or arguments try `dogapi tag --help` for help", false);
         }
     }
 }