onSaveUser event - cannot redirect or access errors
As your code is running during an event, returning the request to the page has to be done a bit more explicitly than when you are performing similar behavior in a controller.

While I'm probably not the best person to explain this in detail, the code below gives one example I was provided in how you can set your route variables and end the request:

if ($errors)
{
    craft()-&gt;urlManager-&gt;setRouteVariables(array(
        'errors' =&gt; $errors
    ));

    $route = craft()-&gt;urlManager-&gt;parseUrl(craft()-&gt;request);
    craft()-&gt;runController($route);
    craft()-&gt;end();
}