When routers are migrated from one Neutron agent to the other, the metering IPtables
rules are removed, which can cause some exceptions that can be ignored. The metering
agent already handled this situation. However, it logs the message as an ERROR, which
can triggers alarms. Therefore, we propose here to change the LOG message from error
to warning.
Closes-Bug: #1904874
Change-Id: I1805a07cef7fc7d7b041e582a4d79fb1a805df71