Break up a circular require between AP/AV

Right now referencing the constant `AbstractController::Rendering`
causes `ActionView::Base` to be loaded, and thus the load hooks for
action_view are run. If that load hook references any part of action
view that then references action controller (such as
`ActionView::TestCase`), the constant `AbstractController::Rendering`
will attempt to be autoloaded and blow up.

With this change, `ActionView::LoadPaths` no longer requires
`ActionView::Base` (which it had no reason to require). There was a
needed class from `AbstractController::Base` in the Rendering module,
which I've moved into its own file so we don't need to load
all of `AbstractController::Base` there.

This commit fixes
rails/rails-controller-testing#21

Author: sgrif

actionpack/lib/abstract_controller/base.rb

@@ -1,13 +1,11 @@
 require 'erubis'
+require 'abstract_controller/error'
 require 'active_support/configurable'
 require 'active_support/descendants_tracker'
 require 'active_support/core_ext/module/anonymous'
 require 'active_support/core_ext/module/attr_internal'
 
 module AbstractController
-  class Error < StandardError #:nodoc:
-  end
-
   # Raised when a non-existing controller action is triggered.
   class ActionNotFound < StandardError
   end

actionpack/lib/abstract_controller/error.rb

@@ -0,0 +1,4 @@
+module AbstractController
+  class Error < StandardError #:nodoc:
+  end
+end

actionpack/lib/abstract_controller/rendering.rb

@@ -1,3 +1,4 @@
+require 'abstract_controller/error'
 require 'active_support/concern'
 require 'active_support/core_ext/class/attribute'
 require 'action_view'

actionview/lib/action_view/view_paths.rb

@@ -1,5 +1,3 @@
-require 'action_view/base'
-
 module ActionView
   module ViewPaths
     extend ActiveSupport::Concern