[WIP] Default messages for \yii\web\*HttpException

Guys it really boring to write every time same messages for exceptions. For example inside controller:

```
public function actionIndex($categoryId)
{
    $category = Category::findOne($categoryId);

    if ($category === null) {
        throw new NotFoundHttpException(Yii::t('app', 'The requested page does not exist.'));
    }
    ...
}
```

Lets create default localizable messages for every `\yii\web\*HttpException` class. With that feature we can forget about writing messages manually in 90% cases. Controller code will look like:

```
public function actionIndex($categoryId)
{
    $category = Category::findOne($categoryId);

    if ($category === null) {
        throw new NotFoundHttpException();
    }
    ...
}
```