btr: Simplify btr_node_status().

This changes btr_node_status() to check for errors before looking
at queue sizes. In certain cases this avoids to call the possibly
expensive btr_get_output_queue_size().

If no more input is going to arrive for an internal node whose output
queue is full, btr_node_status() now returns EOF immediately, which
is better than the previous scheme where we waited for the output
queue to become empty before returning EOF.