render_exception

# File actionpack/lib/action_dispatch/middleware/show_exceptions.rb, line 36
      def render_exception(request, exception)
        backtrace_cleaner = request.get_header "action_dispatch.backtrace_cleaner"
        wrapper = ExceptionWrapper.new(backtrace_cleaner, exception)
        status  = wrapper.status_code
        request.set_header "action_dispatch.exception", wrapper.unwrapped_exception
        request.set_header "action_dispatch.original_path", request.path_info
        request.set_header "action_dispatch.original_request_method", request.raw_request_method
        fallback_to_html_format_if_invalid_mime_type(request)
        request.path_info = "/#{status}"
        request.request_method = "GET"
        response = @exceptions_app.call(request.env)
        response[1]["X-Cascade"] == "pass" ? pass_response(status) : response
      rescue Exception => failsafe_error
        $stderr.puts "Error during failsafe response: #{failsafe_error}\n  #{failsafe_error.backtrace * "\n  "}"

        [500, { "Content-Type" => "text/plain" },
          ["500 Internal Server Error\n"            "If you are the administrator of this website, then please read this web "            "application's log file and/or the web server's log file to find out what "            "went wrong."]]
      end