added missing namespace on Exception in transaction docs

fixes #9429

docs/guide/db-dao.md

@@ -365,7 +365,7 @@ the changes made by the queries prior to that failed query in the transaction.
 ### Specifying Isolation Levels <span id="specifying-isolation-levels"></span>
 
 Yii also supports setting [isolation levels] for your transactions. By default, when starting a new transaction,
-it will use the isolation level set by your database system. You can override the default isolation level as follows,
+it will use the default isolation level set by your database system. You can override the default isolation level as follows,
 
 ```php
 $isolationLevel = \yii\db\Transaction::REPEATABLE_READ;
@@ -429,13 +429,15 @@ try {
     try {
         $db->createCommand($sql2)->execute();
         $innerTransaction->commit();
-    } catch (Exception $e) {
+    } catch (\Exception $e) {
         $innerTransaction->rollBack();
+        throw $e;
     }
 
     $outerTransaction->commit();
-} catch (Exception $e) {
+} catch (\Exception $e) {
     $outerTransaction->rollBack();
+    throw $e;
 }
 ```
 

framework/db/Transaction.php

@@ -18,17 +18,18 @@ use yii\base\InvalidConfigException;
  * The following code is a typical example of using transactions (note that some
  * DBMS may not support transactions):
  *
- * ~~~
+ * ```php
  * $transaction = $connection->beginTransaction();
  * try {
  *     $connection->createCommand($sql1)->execute();
  *     $connection->createCommand($sql2)->execute();
  *     //.... other SQL executions
  *     $transaction->commit();
- * } catch (Exception $e) {
+ * } catch (\Exception $e) {
  *     $transaction->rollBack();
+ *     throw $e;
  * }
- * ~~~
+ * ```
  *
  * @property boolean $isActive Whether this transaction is active. Only an active transaction can [[commit()]]
  * or [[rollBack()]]. This property is read-only.