Update lib/files/cache/legacy.php

Fixing a bug that occurs when trying to update a non-existent 'fscache' table in the database - if unfixed upgrading does not succeed.

Unfortunately I could not get a call to PEAR::isError working. That is why I check for the existence of a property present in class MDB2_Error.
Merge pull request #1634 from irgsmirx/patch-6

Update/Bugfix lib/files/cache/legacy.php during database upgrade