Code review comment for lp:~rainct/libzeitgeist/986230

There's still one issue left - if the cancellable is cancelled, _finish needs to throw IO_ERROR_CANCELLED even if it wanted to throw the log_error. Ideally we'd just use g_simple_async_result_set_check_cancellable, but that's available only from 2.32, so we need to implement it manually.