For consistency send status and messages using the same methods as other ajax requests.  
A user message can be sent with $this->addMessage('success', 'message text');
and setting $this->setLayoutVar('status', 'error'); or $this->setLayoutVar('status', 'success');
is the way to check for errors.  Then you don't have to check the response in javascript errors will get caught by applicants_grid.controller on line 5-18.