To alter the output (enqueueMessage)

Proposal.
To alter the output of system messages (enqueueMessage).
Principle - the string concatenation, and only after that output.
Why:
The latter conclusion HIKASHOP_SUCC_SAVED / ERROR are overwrite the previous output message(s).
That is, messages MENU_WITHOUT_ASSOCIATED_MODULE and others simply lose their meaning - they are not visible.

I'm not sure I understand the problem. Could you explain again please ?

eg.
back\classes\menus.php
if($element=='category' && empty($element)){
$app =& JFactory::getApplication();
$app->enqueueMessage(JText::_('MENU_WITHOUT_ASSOCIATED_MODULE'), 'error');
$app->enqueueMessage(JText::_('ASSOCIATED_MODULE_NEEDED'));
$app->enqueueMessage(JText::sprintf('ADD_MODULE_AUTO',hikashop::completeLink('menus&task=add_module&cid='.(int)$module->id.'&'.JUtility::getToken().'=1')));
}
.....
$app->enqueueMessage(JText::_( 'HIKASHOP_SUCC_SAVED' ));

MENU_WITHOUT_ASSOCIATED_MODULE, ASSOCIATED_MODULE_NEEDED, ADD_MODULE_AUTO displayed 0.00001 sec, then HIKASHOP_SUCC_SAVED displayed.

This is understandable

After the line :
if($element=='category' && empty($element)){

you can add:
$result=false;

which will result in displaying the error message instead of the successful message.

Nothing has changed

Could you copy/paste the code you have now ? That should have removed the successful message and replaced it with the error message when the associated module is missing.