Clobber/delete destination ftp files as needed

To make this transaction atomic you need complete the rename below before deleting the existing (renamed) file.  That means at least one try / except / finally block.

You can do this, but since this is not flying on a spacecraft, I'm ok with relaxing a bit: cut lines 122 and 123 (`ftp_del_file = ..` and `ftp.rename("{}...`) and just `ftp.delete("{}/{}".format(ftp_dirname, file_base)`.  

I'm also bothered that the only exception we would be handling correctly is a failed rename, when in reality I suppose any of the ftp commands could fail for unexpected reasons which might render it impossible to get any more ftp commands through.