Show kernel restarted/died messages.

def kernel_restarted_message(self, msg):
        """Show kernel restarted/died messages."""
        if not self.is_error_shown:
            # If there are kernel creation errors, jupyter_client will
            # try to restart the kernel and qtconsole prints a
            # message about it.
            # So we read the kernel's stderr_file and display its
            # contents in the client instead of the usual message shown
            # by qtconsole.
            try:
                stderr = self._read_stderr()
            except Exception:
                stderr = None
            if stderr:
                self.show_kernel_error('<tt>%s</tt>' % stderr)
        else:
            self.shellwidget._append_html("<br>%s<hr><br>" % msg,
                                          before_prompt=False)