Fix onebox loading on every keystroke after a request fails.

app/controllers/onebox_controller.rb

@@ -4,7 +4,16 @@ class OneboxController < ApplicationController
 
   def show
     Oneboxer.invalidate(params[:url]) if params[:refresh].present?
-    render text: Oneboxer.preview(params[:url])
+
+    result = Oneboxer.preview(params[:url])
+    result.strip! if result.present?
+
+    # If there is no result, return a 404
+    if result.blank?
+      render nothing: true, status: 404
+    else
+      render text: result
+    end
   end
 
 end