Send plain errors for non HTML resources

Without this we throw a second error when we can't find a view
of the correct format and issue a 500 response.

app/controllers/errors_controller.rb

@@ -4,14 +4,23 @@ class ErrorsController < ApplicationController
   skip_authorization_check
 
   def forbidden
-    render :status => :forbidden
+    respond_to do |format|
+      format.html { render :status => :forbidden }
+      format.any { render :status => :forbidden, :plain => "" }
+    end
   end
 
   def not_found
-    render :status => :not_found
+    respond_to do |format|
+      format.html { render :status => :not_found }
+      format.any { render :status => :not_found, :plain => "" }
+    end
   end
 
   def internal_server_error
-    render :status => :internal_server_error
+    respond_to do |format|
+      format.html { render :status => :internal_server_error }
+      format.any { render :status => :internal_server_error, :plain => "" }
+    end
   end
 end