Issue #2189985 by tim.plunkett: Rename UnknownPluginException to PluginNotFoundException.

core/lib/Drupal/Component/Plugin/DefaultPluginBag.php

...al/Component/Plugin/Exception/PluginNotFoundException.php

...pal/Component/Plugin/Exception/UnknownPluginException.php

core/lib/Drupal/Core/Entity/EntityManager.php

core/modules/image/image.module

...dules/image/lib/Drupal/image/Form/ImageEffectFormBase.php

...ts/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php

core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php