net: shell: Handle ENETUNREACH for IPv4 ping

This commit updates the the IPv4 ping command to handle the ENETUNREACH
error number returned by `net_icmpv4_send_echo_request` function when
IPv4 is unavailable.

The `net_icmpv4_send_echo_request` function previously returned EINVAL
when IPv4 is unavailable and this caused the shell command to report
"Invalid IP address" even when the provided IP address is correct; this
problem was corrected by returning ENETUNREACH instead of EINVAL in the
aforementioned function.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>