Don't emit confusing error in netns-cleanup

Don't emit confusing error in netns-cleanup

If we're trying to delete a dhcp/qrouter device with use_veth
= False (Which is the default for some time), we'll first
try to 'ip link del %s', which will fail and emit a confusing
error, then try 'ovs-vsctl del-port'. There's no need to
log an error in such a case.

The patch attempts to future proof by setting the
set_log_fail_as_error(False) to be as tight as possible, so we
do log errors in case the device is somehow used in the future.

Change-Id: I1954bde3ee9a2e43d7615717134b61c5fa7cfbb1
Closes-Bug: #1463800