Adding different error messages for connection failure with IPv6 (#276).

src/net/socket_msg.h

@@ -61,6 +61,8 @@
 #define ERR_SOCK_TRANSFER_INVALID_URL	26
 #define ERR_SOCK_TRANSFER_SELECT_FAILED	27
 #define ERR_SOCK_TRANSFER_FAILED		28
+#define ERR_SOCK_CONNECT_IPV6_FAILED	29
+#define ERR_SOCK_CONNECT_IPV6_TIMEOUT	30
 // The following errors are game errors.
 #define ERR_NET_VERSION_NOT_SUPPORTED	101
 #define ERR_NET_SERVER_MAINTENANCE		102