take the silence operator into account

When the silence operator is used, the error_reporting level is set to 0. Take it into account and don't send anything to Sentry.

This happens for instance [when the cache gets cleared](https://github.com/symfony/symfony1/blob/1.4/lib/cache/sfFileCache.class.php#L98). There are several calls to unlink with a silence operator before that generate tons of Sentry notifications.