MAGETWO-30801: Asynchronous products addition to cart

- merge fix

Author: Pavlo Cherniavskyi

app/code/Magento/ConfigurableProduct/Model/Product/Type/Configurable.php

@@ -774,7 +774,7 @@ protected function _prepareProduct(\Magento\Framework\Object $buyRequest, $produ
                     }
 
                     if (!isset($_result[0])) {
-                        return __('Cannot add the item to shopping cart');
+                        return __('Cannot add the item to shopping cart')->render();
                     }
 
                     /**
@@ -809,7 +809,7 @@ protected function _prepareProduct(\Magento\Framework\Object $buyRequest, $produ
             }
         }
 
-        return (string)$this->getSpecifyOptionMessage();
+        return $this->getSpecifyOptionMessage()->render();
     }
 
     /**

app/code/Magento/Downloadable/Model/Product/Type.php

@@ -485,7 +485,7 @@ protected function _prepareProduct(\Magento\Framework\Object $buyRequest, $produ
             return $result;
         }
         if ($this->getLinkSelectionRequired($product) && $this->_isStrictProcessMode($processMode)) {
-            return __('Please specify product link(s).');
+            return __('Please specify product link(s).')->render();
         }
         return $result;
     }

app/code/Magento/GroupedProduct/Model/Product/Type/Grouped.php

@@ -364,7 +364,7 @@ protected function _prepareProduct(\Magento\Framework\Object $buyRequest, $produ
                             }
 
                             if (!isset($_result[0])) {
-                                return __('We cannot process the item.');
+                                return __('We cannot process the item.')->render();
                             }
 
                             if ($isStrictProcessMode) {
@@ -402,7 +402,7 @@ protected function _prepareProduct(\Magento\Framework\Object $buyRequest, $produ
             }
         }
 
-        return __('Please specify the quantity of product(s).')->__toString();
+        return __('Please specify the quantity of product(s).')->render();
     }
 
     /**