render_exception

# File actionpack/lib/action_dispatch/middleware/show_exceptions.rb, line 42
    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.exception
      request.set_header "action_dispatch.original_path", request.path_info
      request.path_info = "/#{status}"
      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  "}"
      FAILSAFE_RESPONSE
    end