post votes: fix error handling.

* Clean up javascript.
* Return HTTP 422 instead of HTTP 500 on "you have already voted for
  this post" errors.
* In json/xml error responses, return the error message in the `message`
  field, not `reason`.
* In json/xml success responses, return the post itself instead of a
  plain `{ success: true }` object.

app/controllers/post_votes_controller.rb

@@ -1,20 +1,20 @@
 class PostVotesController < ApplicationController
   before_action :voter_only
   skip_before_action :api_check
+  respond_to :js, :json, :xml
+  rescue_with PostVote::Error, status: 422
 
   def create
     @post = Post.find(params[:post_id])
     @post.vote!(params[:score])
-  rescue PostVote::Error, ActiveRecord::RecordInvalid => x
-    @error = x
-    render status: 500
+
+    respond_with(@post)
   end
 
   def destroy
     @post = Post.find(params[:post_id])
     @post.unvote!
-  rescue PostVote::Error => x
-    @error = x
-    render status: 500
+
+    respond_with(@post)
   end
 end