Improve note copying validations and errors

fixes #1956; fixes #1957

app/controllers/posts_controller.rb

@@ -88,7 +88,13 @@ class PostsController < ApplicationController
     @post = Post.find(params[:id])
     @other_post = Post.find(params[:other_post_id].to_i)
     @post.copy_notes_to(@other_post)
-    render :nothing => true
+    
+    if @post.errors.any?
+      @error_message = @post.errors.full_messages.join("; ")
+      render :json => {:success => false, :reason => @error_message}.to_json, :status => 400
+    else
+      head :no_content
+    end
   end
 
   def unvote