client: Send back SOCKS extended error F6 in case of bad hostname

With the new awesome TB 9.5a11, the SocksPort ExtendedErrors are being handled.

Passing an invalid v3 address that is of the right length in bytes, will send back the F6 error but not for an invalid address like: asijdijasdoijqwoieqw.onion.

Problem is that we only send back the F6 code if the address was identified, by length, as a v3. We should handle the BAD_HOSTNAME error code from parse_extended_hostname() and thus send back that extended error.