make better exception

extension/Classes/Core/Form/FormAsFile.php

@@ -622,6 +622,7 @@ class FormAsFile
 
         // Remove columns: id, name, fileStats
         $formId = $form[F_ID];
+        $formName = $form[F_NAME];
         unset($form[F_ID]);
         unset($form[F_NAME]);
         unset($form[F_FILE_STATS]);
@@ -631,12 +632,12 @@ class FormAsFile
         $formElements = $database->sql($sql, ROW_REGULAR, $parameterArray); // array(array(column name => value))
 
         // Translate container references (id to name) and remove all id columns
-        $containerNames = array_reduce($formElements, function ($result, $formElement) {
+        $containerNames = array_reduce($formElements, function ($result, $formElement) use ($formName) {
             if ($formElement[FE_CLASS] === FE_CLASS_CONTAINER) {
                 $containerName = $formElement[FE_NAME];
                 if (in_array($containerName, $result) || $containerName === '') {
                     throw new \UserFormException(json_encode([
-                        ERROR_MESSAGE_TO_USER => 'Duplicate container names.',
+                        ERROR_MESSAGE_TO_USER => "Duplicate container names in form $formName",
                         ERROR_MESSAGE_TO_DEVELOPER => "Container Form Elements must have a unique and nonempty name. Container name: '$containerName'."]),
                         ERROR_FORM_INVALID_NAME);
                 }