Issue #2225353 by tim.plunkett: Convert to an object and provide methods like setError().

core/includes/ajax.inc

core/includes/batch.inc

core/includes/form.inc

core/includes/install.core.inc

core/lib/Drupal/Core/Action/ConfigurableActionBase.php

core/lib/Drupal/Core/Condition/ConditionPluginBase.php

core/lib/Drupal/Core/Config/Entity/DraggableListBuilder.php

core/lib/Drupal/Core/Controller/FormController.php

core/lib/Drupal/Core/Display/VariantBase.php

core/lib/Drupal/Core/Entity/ContentEntityConfirmFormBase.php

core/lib/Drupal/Core/Entity/ContentEntityForm.php

core/lib/Drupal/Core/Entity/ContentEntityFormInterface.php

...Drupal/Core/Entity/Display/EntityFormDisplayInterface.php

core/lib/Drupal/Core/Entity/EntityConfirmFormBase.php

core/lib/Drupal/Core/Entity/EntityForm.php

core/lib/Drupal/Core/Entity/EntityFormBuilder.php

core/lib/Drupal/Core/Entity/EntityFormBuilderInterface.php

core/lib/Drupal/Core/Entity/EntityFormInterface.php

core/lib/Drupal/Core/Field/FieldItemBase.php

core/lib/Drupal/Core/Field/FieldItemInterface.php