modelling: more readable error messages
see https://relaxng.org/jclark/derivative.html#Error_handling for inspiration
an implementation of that is this https://relaxng.org/jclark/jing.html
cc @llhotka
see https://relaxng.org/jclark/derivative.html#Error_handling for inspiration
an implementation of that is this https://relaxng.org/jclark/jing.html
cc @llhotka