fails:Net::FTP#rmdir sends the RMD command with the passed pathname to the server
fails:Net::FTP#rmdir returns nil
fails:Net::FTP#rmdir raises a Net::FTPPermError when the response code is 500
fails:Net::FTP#rmdir raises a Net::FTPPermError when the response code is 501
fails:Net::FTP#rmdir raises a Net::FTPPermError when the response code is 502
fails:Net::FTP#rmdir raises a Net::FTPTempError when the response code is 421
fails:Net::FTP#rmdir raises a Net::FTPPermError when the response code is 530
fails:Net::FTP#rmdir raises a Net::FTPPermError when the response code is 550
