Nope, step-by-step and breakpoint debugging is currently not possible in the UE scripting engine. You have to rely on techniques as implemented by Mofi in his scripts.
for Firefox is the most capable debugger I have come across. Maybe there are others even more capable that I just isn't aware of.
Sadly I can't see how this can integrate with the scripting engine in UE. But if the UE users continue to develop more and more complex scripting solutions, we might want to ask IDM to consider supplying an add-on debugging tool for UE, or some kind of integration with existing tools.