fix: handle authentication failure in sessions controller

Author: cskartikey

app/controllers/sessions_controller.rb

@@ -26,7 +26,8 @@ def create
       reset_session
       session[:user_id] = user.id
       redirect_to root_path, notice: "Signed in with Slack"
-      nil
+    else
+      redirect_to root_path, alert: "Authentication failed or user already signed in"
     end
   end