ange-ftp-potential-error-msgs

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

"^ftp: connect to address .*: No route to host"

Documentation:
Regular expression matching FTP messages that can indicate serious errors.
These mean that something went wrong, but they may be followed by more
messages indicating that the error was somehow corrected.

You can customize this variable.