tools: suppress unuseful warnings during restarting frr

tools: suppress unuseful warnings during restarting frr

Problem:
During restarting frr.service, it throws annoying warnings:
Cannot stop bgpd(and others): pid file not found.

Root Cause:
During restarting process, systemd uses "stop", and watchfrr
uses "restart".
Yes, watchfrr using "restart" is to avoid systemd failing to stop.
But it should be quiet.

Fix:
During restarting service, suppress these warnings from watchfrr.

Signed-off-by: anlan_cs <vic.lan@pica8.com>