Wrap up missing helper exceptions

The `path` method on missing helper errors is inconsistent with the
implementation on LoadError in Ruby 2.0.  Wrap up the missing helper
exceptions so that the inconsistent behavior is mirrored in Ruby 2.0
(until we can figure out *why* it's inconsistent).

actionpack/lib/abstract_controller/helpers.rb

@@ -135,8 +135,7 @@ def modules_for_helpers(args)
             begin
               require_dependency(file_name)
             rescue LoadError => e
-              e.instance_variable_set(:@path, "helpers/#{e.path}")
-              raise
+              raise MissingHelperError.new(e, file_name)
             end
             file_name.camelize.constantize
           when Module
@@ -147,6 +146,15 @@ def modules_for_helpers(args)
         end
       end
 
+      class MissingHelperError < LoadError
+        def initialize(error, path)
+          @error = error
+          @path  = "helpers/#{path}.rb"
+          set_backtrace error.backtrace
+          super("Missing helper file helpers/%s.rb" % error)
+        end
+      end
+
       private
       # Makes all the (instance) methods in the helper module available to templates
       # rendered through this controller.