Consider splitting validation about strings for passed/failed status

[nyalldawson]

Current the validation_summary_report.json contains a single string which describes why a rule may have failed, eg "If True - a feature is found that does not lie on the specified layer.".

The results would be easier for a user to interpret if the json split this string into a passed/failed version for presentation to the user, eg

"feature_not_on_layers_failed": "A feature was found that does not lie on the specified layer.",
"feature_not_on_layers_passed": "No features were found that do not lie on the specified layer."