mail: Fix the error message while sending a email
mail: Fix the error message while sending a email

"Failed to send (a) notification message(s)" was incorrect because it is
possible that emails were delivered successfully even if an exception
occured.

yona-projects/yona,bloodybear/yona,ihoneymon/yobi,doortts/fork-yobi,naver/yobi,oolso/yobi,brainagenet/yobi,bloodybear/yona,yona-projects/yona,doortts/forked-for-history,ahb0327/yobi,Limseunghwan/oss,doortts/fork-yobi,Limseunghwan/oss,ahb0327/yobi,doortts/forked-for-history,bloodybear/yona,ahb0327/yobi,naver/yobi,oolso/yobi,naver/yobi,doortts/fork-yobi,doortts/forked-for-history,bloodybear/yona,doortts/fork-yobi,brainagenet/yobi,Limseunghwan/oss,brainagenet/yobi,yona-projects/yona,ihoneymon/yobi,yona-projects/yona,ihoneymon/yobi,oolso/yobi