Mailer: Adding fromName to config and easier debug for mail options.

There are 2 features in this this PR:
- first commit allows adding a `fromName` option to mailer config:
```php
return [
    'mailer' => [
        'from'     => 'santa@gmail.com',
        'fromName' => 'Santa Claus',
    ],
];
```
- the second one allows easier debug for mailer options. It tries to submit testing email allowing PHPMailer to throw exceptions and then shows up the exception message to user. It's much easier to see now what's wrong (host unavailable, user unavailable, password is wrong).

sorry for submitting these two in one PR