Require explicit adapter/serializer to render JSON API errors

- Separate collection errors from resource errors in adapter
- Refactor to ErrorsSerializer; first-class json error methods
- DOCS
- Rails 4.0 requires assert exact exception class, boo

lib/active_model/serializer/error_serializer.rb

@@ -1,2 +1,6 @@
 class ActiveModel::Serializer::ErrorSerializer < ActiveModel::Serializer
+  # @return [Hash<field_name,Array<error_message>>]
+  def as_json
+    object.errors.messages
+  end
 end

lib/active_model/serializer/errors_serializer.rb

@@ -0,0 +1,23 @@
+require 'active_model/serializer/error_serializer'
+class ActiveModel::Serializer::ErrorsSerializer < ActiveModel::Serializer
+  include Enumerable
+  delegate :each, to: :@serializers
+  attr_reader :object, :root
+
+  def initialize(resources, options = {})
+    @root = options[:root]
+    @object = resources
+    @serializers = resources.map do |resource|
+      serializer_class = options.fetch(:serializer) { ActiveModel::Serializer::ErrorSerializer }
+      serializer_class.new(resource, options.except(:serializer))
+    end
+  end
+
+  def json_key
+    nil
+  end
+
+  protected
+
+  attr_reader :serializers
+end