如果mySQL数据库的更新失败，如何收到错误消息？

I want to get a simple error message, when the update of my SQL database failed.

foreach($animals as $row) {

   $cat = "234";

   $sql = "UPDATE animals set cat2 = ?  WHERE cat1 = ?";
   $q = $pdo->prepare($sql);
   $q->execute(array($cat,$cat));                               
}

if ($q->execute(array($cat,$cat)))
{
echo "success";
}
else
{
echo "failure";
}

But even if the update failed and nothing is updated, I get the message: success.