The error codes probably correspond with curl exit codes.
6 Could not resolve host. The given remote host could not be resolved.
7 Failed to connect to host.
8 Weird server reply. The server sent data curl could not parse.
…
15 FTP cannot use host. Could not resolve the host IP we got in the 227-line.
which mostly look like host lookup-related issues. As for why 8 and not 13 or 14 which deals with strange FTP responses…no idea.