feat(internal): add function to stub errors with a toJSON method

lib/LogMessage.js

@@ -220,6 +220,35 @@ class LogMessage {
       )
     );
   }
+
+  /**
+   * Stubs an Error or Error-like object to include a toJSON method.
+   * @static
+   * @param {Error} err An Error or Error-like object.
+   * @return {Error}
+   */
+  static stubError(err) {
+    err.toJSON = function () {
+      const keys = [
+        'name',
+        'message',
+        'stack'
+      ].concat(Object.keys(err));
+
+      return keys.reduce((obj, key) => {
+        if(Object.prototype.hasOwnProperty.call(err, key)) {
+          const val = err[key];
+          if(typeof val !== 'function') {
+            obj[key] = val;
+          }
+        }
+
+        return obj;
+      }, {});
+    };
+
+    return err;
+  }
 }
 
 LogMessage.symbols = symbols;