13 files declare their use of Error in Drupal 9

bootstrap.inc in core/includes/bootstrap.inc

Functions that need to be loaded on every Drupal request.

BrowserHtmlDebugTrait.php in core/tests/Drupal/Tests/BrowserHtmlDebugTrait.php

BrowserTestBase.php in core/tests/Drupal/Tests/BrowserTestBase.php

DefaultExceptionHtmlSubscriber.php in core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php

errors.inc in core/includes/errors.inc

Functions for error handling.

ErrorTest.php in core/tests/Drupal/Tests/Core/Utility/ErrorTest.php

ExceptionLoggingSubscriber.php in core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php

ExceptionTestSiteSubscriber.php in core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php

FinalExceptionSubscriber.php in core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php

MigrateExecutable.php in core/modules/migrate/src/MigrateExecutable.php

SessionHandler.php in core/lib/Drupal/Core/Session/SessionHandler.php

TestHttpClientMiddleware.php in core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php

update.inc in core/includes/update.inc

Drupal database update API.