Properly handle failure to update times

Linux won't allow utime() to be called on a file that's not *owned* by
the current user (even if it is writable by him). This causes a problem
for people running their torrent software as one user, but tvnamer as
another.

See http://dbr.lighthouseapp.com/projects/36049/tickets/130-After-Recent-tvnamer-Update for an example.