    # File lib/phusion_passenger/utils.rb, line 857
            def backtrace_string(current_location = nil)
                    if current_location.nil?
                            location = nil
                    else
                            location = "in #{current_location} "
                    end
                    return "*** Exception #{self.class} #{location}" <<
                            "(#{self}) (process #{$$}, thread #{Thread.current}):\n" <<
                            "\tfrom " << backtrace.join("\n\tfrom ")
            end