Add more console output during worker pool initializiation.

I had yet another no-CPU hang and want to be able to narrow down the cause next time.
Fix a line_boundaries() crash when there aren't any tags.
Filter out zero-length spans.

They can cause starts to sort after ends, crashing the tag balancer.
Tolerate None start- and endpoints.

Some plugin really still is spitting these out.
Fix an UnboundLocal error for `id` when formatting an exception.
Fix balancing of certain line-spanning tags.

Special-case Lines more vigorously in balance_tags_with_empties().

This removes the side effect that was causing Line tags (and only Line tags) to become misordered in relation to other coincident tags, which we were then making up for in html_lines(). However, that led to a failure to properly close all tags before the end of a line in some cases. Now, balanced_tags_with_empties() encloses each line in properly-balanced Line tags, without_empty_tags() preserves them, and html_lines() uses the line-end tag as impetus to flush its buffer.