Patch Review Request: #1921 Plugin Loader lists all plugins as missing when only one is missing

I've patched the plugin loader and related initializer test to resolve
a problem I ran into when upgrading a Rails 1.2.6 app to 2.2.2. I
would appreciate some folks trying it out. This is a quickie.

The problem: When starting up the console or server I kept getting a
LoadError message stating that all of my plugins were missing. It
looks like this: "Could not locate the following plugins:
acts_as_audited, acts_as_geocodable, acts_as_list, ...[long list of
plugins]". After some debugging it turned out I actually had just one
plugin missing, despite the lengthy list in the message.

This patch changes #ensure_all_registered_plugins_are_loaded! to list
exactly the plugins that could not be loaded, rather than listing all
plugins. I believe this is what the method was originally intended to

Has anyone else run into this? A typo in a plugin name within your
configuration.plugins list is sufficient to trigger it.