Raise exception if group not found/not readable by a user

ee/app/finders/epics_finder.rb

@@ -4,8 +4,7 @@ class EpicsFinder < IssuableFinder
   end
 
   def execute
-    raise ArgumentError, 'group_id argument is missing' unless params[:group_id]
-    return [] unless group
+    raise ArgumentError, 'group_id argument is missing' unless group
 
     items = init_collection
     items = by_created_at(items)

ee/app/policies/ee/group_policy.rb

@@ -30,7 +30,10 @@ module EE
         enable :destroy_epic
       end
 
-      rule { auditor }.enable :read_group
+      rule { auditor }.policy do
+        enable  :read_group
+        enable  :read_epic
+      end
       rule { admin }.enable :read_epic
       rule { has_projects }.enable :read_epic