feat: add a reference to the node a rule failed on

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.

lib/core/base/audit.js

@@ -405,7 +405,10 @@ 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 reference to the node the rule failed on for easier debugging.
+						// See https://github.com/dequelabs/axe-core/issues/1317.
+						node: err.node || null
 					});
 					// resolve
 					resolve(errResult);

lib/core/base/check.js

@@ -84,6 +84,8 @@ Check.prototype.run = function(node, options, context, resolve, reject) {
 				context
 			);
 		} catch (e) {
+			// Save a reference to the node we errored on for futher debugging.
+			e.node = node;
 			reject(e);
 			return;
 		}