Create SumoLogic alert for missing notification for mentioned user

[vikasrohit]

We need to setup the sumo logic alert to detect presence of Unable to send notification to mentioned user to alert about the situation where we are skipping the notification to a user who is mentioned inside a post after fixes made in #128.

fyi @sachin-maheshwari @mtwomey @RishiRajSahu

[vikasrohit]

We need to add Unable to find user with handle to detect failure of sending notifications to users for whom we are unable to find details from member search.