Line number in error message about unresolved include or include tag is off by one
The line number reported in the error message when an include target or tag cannot be resolved is off by one. This happens because Asciidoctor core hands control over the the custom include processor after consuming the line with the include directive. Thus, the method reader.$cursor_at_prev_line() must be used instead of reader.getCursor().
(If this logic were to change in core, it would be necessary to revert back to using reader.getCursor()).