net: ip: change some error messages to NET_ERR

Several error messages are currently being logged as NET_DBG which
requires the user to have CONFIG_SYS_LOG_NET_LEVEL=4.

Let's show these as errors so they are more visible.

Change-Id: I28c9a1aedb78787ef098a9bf565472a437373933
Signed-off-by: Michael Scott <michael.scott@linaro.org>