Issue #2239299 by tim.plunkett: Form errors should only be set during validation.

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

...les/aggregator/lib/Drupal/aggregator/Form/OpmlFeedAdd.php

...odules/config/lib/Drupal/config/Form/ConfigImportForm.php

core/modules/file/file.module

...ests/file_test/lib/Drupal/file_test/Form/FileTestForm.php

core/modules/locale/locale.bulk.inc

...modules/search/lib/Drupal/search/Form/SearchBlockForm.php

core/modules/shortcut/shortcut.admin.inc

...dules/system/lib/Drupal/system/Form/ThemeSettingsForm.php

core/modules/update/update.manager.inc