Catch DiskFileNotExist exceptions when attempting to purge files. The
file may have passed its reclaim age since being reverted and will be
cleaned up when the reconstructor opens it for purging, raising a
DiskFileNotExist. The exception is OK - the diskfile was about to be
purged.
Change-Id: I5dfdf5950c6bd7fb130ab557347fbe959270c6e9