feat: add a reference to the node a rule failed on (#1321)

This patch adds a `.node` reference to an errored rule result. This can be used to determine which node the rule errored on and will be helpful for debugging purposes.

Closes #1317.

## Reviewer checks

**Required fields, to be filled out by PR reviewer(s)**
- [x] Follows the commit message policy, appropriate for next version
- [x] Has documentation updated, a DU ticket, or requires no documentation change
- [x] Includes new tests, or was unnecessary
- [x] Code is reviewed for security by: @WilcoFiers

lib/core/base/audit.js

@@ -391,7 +391,10 @@ function getDefferedRule(rule, context, options) {
 						description: 'An error occured while running this rule',
 						message: err.message,
 						stack: err.stack,
-						error: err
+						error: err,
+						// Add a serialized reference to the node the rule failed on for easier debugging.
+						// See https://github.com/dequelabs/axe-core/issues/1317.
+						errorNode: err.errorNode
 					});
 					// resolve
 					resolve(errResult);

lib/core/base/check.js

@@ -1,4 +1,4 @@
-/*global CheckResult */
+/*global CheckResult,DqElement */
 
 function createExecutionContext(spec) {
 	/*eslint no-eval:0 */
@@ -58,6 +58,7 @@ Check.prototype.enabled = true;
  * @param  {Function} callback Function to fire when check is complete
  */
 Check.prototype.run = function(node, options, context, resolve, reject) {
+	/* eslint max-statements: ["error", 17] */
 	'use strict';
 	options = options || {};
 	var enabled = options.hasOwnProperty('enabled')
@@ -84,6 +85,13 @@ Check.prototype.run = function(node, options, context, resolve, reject) {
 				context
 			);
 		} catch (e) {
+			// In the "Audit#run: should run all the rules" test, there is no `node` here. I do
+			// not know if this is intentional or not, so to be safe, we guard against the
+			// possible reference error.
+			if (node && node.actualNode) {
+				// Save a reference to the node we errored on for futher debugging.
+				e.errorNode = new DqElement(node.actualNode).toJSON();
+			}
 			reject(e);
 			return;
 		}

test/core/base/audit.js

@@ -1035,9 +1035,10 @@ describe('Audit', function() {
 					throw err;
 				}
 			});
-
+			axe._tree = axe.utils.getFlattenedTree(fixture);
+			axe._selectorData = axe.utils.getSelectorData(axe._tree);
 			a.run(
-				{ include: [axe.utils.getFlattenedTree(fixture)[0]] },
+				{ include: [axe._tree[0]] },
 				{
 					runOnly: {
 						type: 'rule',