load plugin in bundles just as they are in Config::MVP::Section, with an...

... exception thrown if the plugin could not be loaded

It duplicates some methods from Config::MVP::Section (ew) that could possibly go somewhere else, but I don't grok your architecture well enough to know where. Also furthers the use of Class::Load (double ew!), but the necessary interface in Module::Runtime doesn't exist yet (on my todo list though!)

With this in place, a 'dzil build' with a pluginbundle referencing an uninstalled plugin will generate the proper error: "Required plugin Dist::Zilla::Plugin::Foo isn't installed. Run 'dzil authordeps'..." rather than an ugly uncaught exception.