Use the generic existing constant

Use the existing constant because the constant is not available in the Magento\CatalogImportExport\Model\Import\Product class.

Author: bartdelvaux

app/code/Magento/CatalogImportExport/Model/Import/Product/Type/AbstractType.php

@@ -270,7 +270,7 @@ public function isRowValid(array $rowData, $rowNum, $isNewProduct = true)
                         ))
                     ) {
                         $this->_entityModel->addRowError(
-                            \Magento\CatalogImportExport\Model\Import\Product::ERROR_VALUE_IS_REQUIRED,
+                            \Magento\CatalogImportExport\Model\Import\Product\RowValidatorInterface::ERROR_VALUE_IS_REQUIRED,
                             $rowNum,
                             $attrCode
                         );