Supply document position for error reporting

[donmendelson]

Markdown parser already supplies document position in parsing exceptions as line number plus character position in line. However, structural anomalies without a syntax error provide no reference. For example, an application may report that the name of an element is missing. There would be no way for a user to know which line in a markdown file caused the problem. Therefore, md-grammar should pass document position to applications even when there is no syntax error.