Display error message on Sphinx rich text failure

def _on_sphinx_thread_error_msg(self, error_msg):
        """ Display error message on Sphinx rich text failure"""
        self._sphinx_thread.wait()
        self.plain_text_action.setChecked(True)
        sphinx_ver = programs.get_module_version('sphinx')
        QMessageBox.critical(self,
                    _('Help'),
                    _("The following error occured when calling "
                      "<b>Sphinx %s</b>. <br>Incompatible Sphinx "
                      "version or doc string decoding failed."
                      "<br><br>Error message:<br>%s"
                      ) % (sphinx_ver, error_msg))