BackupHelper tasks will never get marked as cancelled

[Charles Kerr]

In this block:

> if (n_uploaded_ == q_ptr->expected_size())
> {
> q_ptr->set_state(Helper::State::COMPLETE);
> }
> else if (read_error_ || write_error_ || n_uploaded_ != q_ptr->expected_size())
> {
> q_ptr->set_state(Helper::State::FAILED);
> }
> else if (cancelled_)
> {
> q_ptr->set_state(Helper::State::CANCELLED);
> }

We'll never reach the third else because any value of n_uploaded will either be
"== q_ptr->expected_size()" or
"!= q_ptr->expected_size()".

As written this block wrongly behaves as:

    q_ptr->set_state(n_uploaded == expected_size() ? COMPLETE : FAILED)