Show possible errors when setting the selected Matplotlib backend.

def _show_mpl_backend_errors(self):
        """
        Show possible errors when setting the selected Matplotlib backend.
        """
        if not self.external_kernel:
            self.shellwidget.silent_execute(
                    "get_ipython().kernel._show_mpl_backend_errors()")
        self.shellwidget.sig_prompt_ready.disconnect(
            self._show_mpl_backend_errors)