Fix wrong yii\db\Exception namespace

src/SaveRelationsBehavior.php

@@ -9,7 +9,7 @@ use yii\base\Exception;
 use yii\base\ModelEvent;
 use yii\base\UnknownPropertyException;
 use yii\db\BaseActiveRecord;
-use Yii\db\Exception as DbException;
+use yii\db\Exception as DbException;
 use yii\db\Transaction;
 use yii\helpers\ArrayHelper;
 use yii\helpers\Inflector;