Clobber/delete destination ftp files as needed

I'm not entirely clear on why it is more atomic to do the final rename before the delete... but maybe I'm just assuming that if you can't delete the temp file you probably ran into other problems here anyway.  Which other exceptions would you like to handle so you might be able to get more ftp commands through and how would that help?

To support being truly robust and atomic I think you'd really need to have rename() check file status in Ska.ftp, but I figured that wasn't my purview.  And regarding errors I think all of the ftp commands can fail with unhandled IOErrors (though I didn't try to make permissions errors) and there's still the unhandled recursive object delete problem if the SFTP isn't correctly initialized.