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/views/post_votes/create.js.erb

@@ -1,9 +1,5 @@
-<% if @error %>
-  $(window).trigger("danbooru:notice", "<%= j @error.to_s %>");
-<% else %>
-  $(window).trigger("danbooru:notice", "Vote saved");
-  $("#score-for-post-<%= @post.id %>").html(<%= @post.score %>);
-<% end %>
+Danbooru.Utility.notice("Vote saved");
+$("#score-for-post-<%= @post.id %>").html(<%= @post.score %>);
 
 $("#vote-links-for-post-<%= @post.id %>").hide();
 $("#unvote-link-for-post-<%= @post.id %>").show();