fails:Net::FTP#system sends the SYST command to the server
fails:Net::FTP#system returns the received information
fails:Net::FTP#system raises a Net::FTPPermError when the response code is 500
fails:Net::FTP#system raises a Net::FTPPermError when the response code is 501
fails:Net::FTP#system raises a Net::FTPPermError when the response code is 502
fails:Net::FTP#system raises a Net::FTPTempError when the response code is 421
