Prevent find in the unmanaged-file inspector from failing

Old Gnome versions did created for every user under ~/.gvfs a mount
point to the GVFS fuse file system.
Root couldn't access these so the inspection did fail.

The find command does give a return code bigger than zero
if it does scan the parent directory while still delivering
the correct output.

To prevent the unmanaged-file inspector from failing the
ExecutionFailed statement is catched and a warning is
shown.

An easy way to reproduce the issue is to run
`/usr/lib64/gvfs/gvfs-fuse-daemon /<user-writeable-directory>` as user.
The related package is gvfs-fuse.

Fixes #393