Improve exception handling in case of NULL from getPluginClass

See #1897364: How the bleepity bleep do you debug plugin-related errors? :) → .

xjm asked me to file this because:

Drupal\Component\Plugin\Exception\PluginException: The plugin (ckeditor) did not specify an instance class. in Drupal\Component\Plugin\Factory\DefaultFactory::getPluginClass() (line 62 of /Users/webchick/Sites/spark/spork/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php).

should apparently have been something more like:

Hey, dummy. That plugin_id returned NULL.