Check for lock existence before unlinking

My logs are filled with a bazillion errors stating "Warning: unlink(/var/www/mysite/app/cache/prod/http_cache/md/cf/47/c693da5dab3eccb65fa36a9b4b07ad0f7cc4.lck): No such file or directory in /var/www/mysite/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpCache/Store.php line 53"
fixed CS