fix ip -force -batch to continue on errors

fix ip -force -batch to continue on errors

This patch replaces exits with returns in several
iproute2 commands. This fixes `ip -batch -force`
to not exit but continue on errors.

$cat c.txt
route del 1.2.3.0/24 dev eth0
route del 1.2.4.0/24 dev eth0
route del 1.2.5.0/24 dev eth0
route add 1.2.3.0/24 dev eth0

$ip -force -batch c.txt
RTNETLINK answers: No such process
Command failed c.txt:2
RTNETLINK answers: No such process
Command failed c.txt:3

Reported-by: Sven-Haegar Koch <haegar@sdinet.de>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>