sd-netlink: read error message for NFNL_MSG_BATCH_BEGIN (#39967)

Before:
```
$ unshare --user --map-users=0:$(id -u):1 --map-groups=0:$(id -g):1 build/test-firewall-util
/* test_v6 */
src/test/test-firewall-util.c:34: Assertion failed: "r = fw_nftables_add_masquerade(nfnl, true, AF_INET6, &u1, 128)" failed with unexpected error: -110/ETIMEDOUT
Aborted (core dumped)
```

After:
```
$ unshare --user --map-users=0:$(id -u):1 --map-groups=0:$(id -g):1 /var/build/test-firewall-util
/* test_v6 */
test-firewall-util: Failed to add IPv6 masquerade, skipping tests: Operation not permitted
/* test_v4 */
test-firewall-util: Failed to add IPv4 masquerade, skipping tests: Operation not permitted
```