Fix error message

When an error is thrown as follows:

```javascript
throw new HTTPError(403);
```

If `error.message` is Object, an error message is displayed as follows.

```
HTTPError: [object Object]
    at ...
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
```

`[object Object]` is not good.

This patch fixed it. An error message will be displayed as follows.

```
HTTPError: Forbidden
    at ...
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
```