Craft 3 Controller - Redirect with errors
In Yii2 there is no such a thing as "redirectWithErrors". Usually you have 2 scenarios


There are validation errors -> display the form again with those errors
Successful -> redirect


The first one doesn't require a redirect. You just return null in your controller and Yii2 will route the to the url specified via GETparameter.

So you can have a form like this. 

&lt;form method="post" action="myUrl"&gt;
    &lt;input type="hidden" name="action" value="mycontroller/save"&gt;
&lt;/form&gt;


Yii2 will do the following


Route to your controller and hit the save action
When you redirect or return a valid object of type yii\web\Response or a string that can be converted into that it will return the Response
When you don't return something it will route the request a second time with your get param myUrl, that can be treated as you routeWithErrors function because you set the errors Craft::$app-&gt;getUrlManager()-&gt;setRouteParams(['model' =&gt; $model]) and have then access to these parameters.


If you really dislike this, you could use return Craft::runAction('controller/action', $parameters); to run a controller action with certain values

Edit 
Usually you want to leave action="myUrl" blank and don't insert it. That way Yii2 will automatically reload the current route so you can dispaly your errors