Collapse JSON API success/failure documents in one adapter

Idea per remear (Ben Mills) in the slack:
https://amserializers.slack.com/archives/general/p1455140474000171

remear:

    just so i understand, the adapter in `render json: resource, status: 422, adapter: 'json_api/error',
    serializer: ActiveModel::Serializer::ErrorSerializer` is a different one than, say what i’ve
    specified in a base serializer with `ActiveModel::Serializer.config.adapter = :json_api`. correct?

    and a followup question of, why not same adapter but different serializer?

me:

   With the way the code is written now, it might be possible to not require a special jsonapi adapter.
   However, the behavior is pretty different from the jsonapi adapter.

   this first draft of the PR had it automatically set the adapter if there were errors.  since that
   requires more discussion, I took a step back and made it explicit for this PR

   If I were to re-use the json api adapter and remove the error one, it think the serializable hash
   method would look like

   ```
   def serializable_hash(options = nil)
     return { errors: JsonApi::Error.collection_errors } if serializer.is_a?(ErrorsSerializer)
     return { errors: JsonApi::Error.resource_errors(serializer) } if serializer.is_a?(ErrorSerializer)
     options ||= {}
   ```

   I suppose it could be something more duckish like

   ```
   def serializable_hash(options = nil)
     if serializer.errors? # object.errors.any? || object.any? {|o| o.errors.any? }
       JsonApi::Error.new(serializer).serializable_hash
     else
       # etc
   ```

lib/active_model/serializer.rb

@@ -118,6 +118,10 @@ module ActiveModel
       end
     end
 
+    def success?
+      true
+    end
+
     # Used by adapter as resource root.
     def json_key
       root || object.class.model_name.to_s.underscore

lib/active_model/serializer/collection_serializer.rb

@@ -22,6 +22,10 @@ module ActiveModel
         end
       end
 
+      def success?
+        true
+      end
+
       def json_key
         root || derived_root
       end

lib/active_model/serializer/error_serializer.rb

@@ -3,4 +3,8 @@ class ActiveModel::Serializer::ErrorSerializer < ActiveModel::Serializer
   def as_json
     object.errors.messages
   end
+
+  def success?
+    false
+  end
 end

lib/active_model/serializer/errors_serializer.rb

@@ -1,5 +1,5 @@
 require 'active_model/serializer/error_serializer'
-class ActiveModel::Serializer::ErrorsSerializer < ActiveModel::Serializer
+class ActiveModel::Serializer::ErrorsSerializer
   include Enumerable
   delegate :each, to: :@serializers
   attr_reader :object, :root
@@ -13,6 +13,10 @@ class ActiveModel::Serializer::ErrorsSerializer < ActiveModel::Serializer
     end
   end
 
+  def success?
+    false
+  end
+
   def json_key
     nil
   end