[HttpKernel] Check for lock existence before unlinking

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

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"