Issue #2230121 by znerol, dawehner | sun: Fixed Remove exit() from FormBuilder.

core/core.services.yml

...rupal/Core/EventSubscriber/DefaultExceptionSubscriber.php

...l/Core/EventSubscriber/EnforcedFormResponseSubscriber.php

core/lib/Drupal/Core/Form/EnforcedResponse.php

core/lib/Drupal/Core/Form/EnforcedResponseException.php

core/lib/Drupal/Core/Form/FormBuilder.php

core/modules/system/src/Tests/Form/ResponseTest.php

...ules/system/tests/modules/form_test/form_test.routing.yml

...les/system/tests/modules/form_test/form_test.services.yml

...form_test/src/EventSubscriber/FormTestEventSubscriber.php

...tests/modules/form_test/src/Form/FormTestResponseForm.php

...ules/form_test/src/StackMiddleware/FormTestMiddleware.php

core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php