[Mailer] Get failing email addresses

**Description**  
Hi, 
I'm looking for a way to display the email addresses to which the message was not sent.

**Example**  
Something like that used to exist on Swiftmailer

```
$failure = [];

        if (!$this->mailer->send($message, $failure)) {
            ....
        } 

```
With `$failure`  containing the emails to which the message was not sent

