Issue #2379871: White Screen of Death on admin/config

For some reason we don't always get a plugin from ctools_get_plugins()
and/or ctools_plugin_get_class(). This looks like a broken cache in
ctools somewhere. The root cause might be
https://www.drupal.org/node/2598166.

This fix introduces a more defensive behaviour around assuming we loaded
a plugin.

We won't avoid errors but will throw a runtime exception instead of
getting a white screen of death.