Mail class, send email error handler

[hellomaya]

From opencart 1.5.6, it deal with email with this logic,

if (!$this->to) {
trigger_error('Error: E-Mail to required!');
exit();
}

So every time if there is something wrong, it will exit the code, that definitely not good, when user registered and used a wrong email perhaps fake one, this will bring code just exit, nothing bug a blank page returned.