samples: net: zperf: Fix only IPv4/only IPv6 build, better messages

The code couldn't really build with only CONFIG_NET_IPV4 or
CONFIG_NET_IPV6 enabled, because some parts weren't covered
by #ifdef's. Regroup the code to get it covered.

Also, improve error/notice messages.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>