Explicitly use PDOException in namespaces.

core/includes/Drupal/Database/Connection.php

@@ -3,6 +3,7 @@
 namespace Drupal\Database;
 
 use PDO;
+use PDOException;
 
 /**
  * Base Database API class.

core/includes/Drupal/Database/DatabaseStatementPrefetch.php

@@ -5,6 +5,7 @@ namespace Drupal\Database;
 use Drupal\Database\Connection;
 use Iterator;
 use PDO;
+use PDOException;
 
 /**
  * An implementation of DatabaseStatementInterface that prefetches all data.