Add condition for deleted file

Not throw RuntimeException: SplFileInfo::getMTime(): stat failed
When the file was already deleted by another parallel process.

@src: http://framework.zend.com/issues/browse/ZF-11858
I belive that this error occurs only under some load, when two (or more) process fire at the same time. One process tries to run SplFileInfo::getMTime() on file wich was already deleted by other parallel process.
Solution for this could be some kind of semaphore or at least an additional file_exists() check.