need to demonstrate error cb not just drain cb

need to demonstrate to users how to use a single error callback

lib/queue.js

@@ -88,6 +88,11 @@ import wrapAsync from './internal/wrapAsync';
  *     console.log('all items have been processed');
  * };
  *
+ * // assign an error callback
+ * q.error = function(err, task) {
+ *     console.error('task experienced an error');
+ * };
+ *
  * // add some items to the queue
  * q.push({name: 'foo'}, function(err) {
  *     console.log('finished processing foo');