[BUGFIX] Show error messages in multistep checkout

Due to the implementation of the PRG pattern the
errors directly next to the errorneous fields were
no longer displayed.

With this commit the PRG pattern will only be
used if no errors occur during the current step.

Fixes: #532

Classes/Controller/Cart/CartController.php

@@ -125,7 +125,10 @@ public function showAction(
             $currentStep = (int)$this->request->getArgument('step');
         }
 
-        if ($currentStep && $this->request->getMethod() === 'POST') {
+        if ($currentStep
+            && $this->request->getMethod() === 'POST'
+            && !$extbaseAttribute->getOriginalRequestMappingResults()->hasErrors()
+        ) {
             return $this->redirect('show', null, null, ['step' => $currentStep])->withStatus(303);
         }
         // Redirect to step 1 if cart is empty.