Issue #2940203 by almaudoh, dawehner: Use dedicated Exception classes for extension system

core/includes/bootstrap.inc

...ore/Extension/Exception/UninstalledExtensionException.php

...al/Core/Extension/Exception/UnknownExtensionException.php

core/lib/Drupal/Core/Extension/ExtensionList.php

core/lib/Drupal/Core/Extension/ModuleHandler.php

core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php

core/lib/Drupal/Core/Extension/ThemeHandler.php

core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php

core/lib/Drupal/Core/Extension/ThemeInstaller.php

core/lib/Drupal/Core/Extension/ThemeInstallerInterface.php

core/modules/system/system.module

...ests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php

core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php

core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php