Check that file exists before deleting it

I frequently see errors like the following when Boost is trying to delete a file that doesn't exist:

unlink(FILENAME_HERE): No such file or directory

The offending code is in boost_expire_cache():

if (unlink($filename)) {
  boost_log('Removed !file from the boost cache.', array('!file' => $filename), WATCHDOG_DEBUG);
}
else {
  boost_log('Could not delete !file from the boost cache. Check file permissions.', array('!file' => $filename), WATCHDOG_WARNING);
}

I propose simply checking if the file exists before attempting to delete it. If it's not there, that's just fine and no warning needs to be thrown or watchdog message logged.