Issue #2971375 by cspitzlay: Fix "does not exists" in comments and test error messages

core/includes/install.core.inc

core/lib/Drupal/Core/Archiver/ArchiveTar.php

...odules/aggregator/tests/src/Functional/DeleteFeedTest.php

...modules/field/src/Plugin/migrate/process/ProcessField.php

core/modules/system/system.module

.../modules/user/src/Plugin/migrate/process/UserLangcode.php

core/tests/Drupal/KernelTests/Core/Database/SchemaTest.php

core/tests/Drupal/Tests/WebAssert.php