Fixed EZP-20813: dfscleanup may delete too much data

Fixes http://jira.ez.no/browse/EZP-20813

### Situation

The `dfscleanup.php` script could, if either NFS or DB became unavailable, delete the whole database or the whole NFS folder. 

### Changes

This PR adds checks availability of both:
- We check that `<NfsMountPath>/<VarDir>` exists when a file was reported as missing
- Database error induced by `eZDFSFileHandler::exists()` and `eZDFSFileHandler::getFileList()`will now throw an Exception if a database error, instead of returning a meaningless false

### Testing done
Added interrupts ( `fgets( STDIN )` ) before critical operations, and:
- made NFS unavailable, simulating a mount error
- stopped the database server
The script now aborts, explaining why, when one of those occur.

### Todo
- [ ] Move the CS only changes to a distinct commit