SMB ignore connection errors when NetBIOS is disabled

When NetBIOS is blocked or disabled, a smb::look() call can still get
the list of shares from smbclient, even though the name resolution at
the end of the command fails.

This fix makes it ignore the connection error if we already got the list
of shares.

Fixed #5860

Please review @karlitschek @DeepDiver1975 @icewind1991 

FYI @ser72