MM-35298: Follow thread when added to channel (#19311)

* MM-35298: Follow thread when added to channel

* return better error if thread doesn't exist

* update test for possible race

* use correct comparision operator

store/sqlstore/thread_store.go

@@ -425,6 +425,9 @@ func (s *SqlThreadStore) GetThreadForUser(teamId string, threadMembership *model
 
 	err := s.GetReplica().SelectOne(&thread, query, args...)
 	if err != nil {
+		if err == sql.ErrNoRows {
+			return nil, store.NewErrNotFound("Thread", threadMembership.PostId)
+		}
 		return nil, err
 	}