server: fix wrong log message
server: fix wrong log message

Fix the regression due to c4fe45902e736a858c4870bcab01c9143551f696

Also switch from Fatal from Warn.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

osrg/gobgp,a16/gobgp,tamihiro/gobgp,fujita/gobgp,a16/gobgp,a16/gobgp,ishidawataru/gobgp,a16/gobgp,osrg/gobgp,j-kato/gobgp,a16/gobgp,fujita/gobgp,j-kato/gobgp,osrg/gobgp,ishidawataru/gobgp,fujita/gobgp,tamihiro/gobgp,osrg/gobgp,fujita/gobgp,a16/gobgp,Dipsingh/gobgp,ishidawataru/gobgp,Dipsingh/gobgp,Dipsingh/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp,tamihiro/gobgp