The current devel error handling options let you choose between the nice backtrace and the standard drupal message area, but I have had situations where I could really have done with both. Some themes do not display the backtrace the same in every situation and depending on what the page is doing it has been very useful to get both. Attached are a couple of screen shots.
I will add the patch too, and you can consider whether you like it. Obviously I will make a D7 patch too, but I was working on a D6 site when I needed this.