ange-ftp-fatal-msgs

ange-ftp-fatal-msgs is a variable defined in `ange-ftp.el'.
Its value is

"^ftp: \\|^Not connected\\|^530 \\|^4[25]1 \\|rcmd: \\|^No control connection\\|unknown host\\|^lost connection"

Documentation:
Regular expression matching FTP messages that indicate serious errors.

These mean that the FTP process should be (or already has been) killed.

You can customize this variable.