I think we were worried that this would end up clobbering other people (e.g. extensions) who might be setting markers in the gutter. Obviously in general we don't have a good story for managing gutter markers yet, so we can't solve conflicts between extensions, but I think we were hoping to avoid actually using the gutter markers ourselves.

Would it be possible to use `addLineClass()` to set a CSS class on the "wrapper" node for lines that have an error, and then add a contextual rule on the line number within line wrapper nodes that have that class set? Since you can have multiple line classes, this shouldn't interfere with other gutter markers.