TTL txn will not return error when rolling back

[lcwangchao]

tidb/ttl/session/session.go

Lines 97 to 113 in 1e7c552

	func (s *session) RunInTxn(ctx context.Context, fn func() error) (err error) {
	tracer := metrics.PhaseTracerFromCtx(ctx)
	defer tracer.EnterPhase(tracer.Phase())
	tracer.EnterPhase(metrics.PhaseBeginTxn)
	if _, err = s.ExecuteSQL(ctx, "BEGIN"); err != nil {
	return err
	}
	tracer.EnterPhase(metrics.PhaseOther)
	success := false
	defer func() {
	if !success {
	_, err = s.ExecuteSQL(ctx, "ROLLBACK")
	terror.Log(err)
	}
	}()

When rollingback, the error will be reset to nil