Fix warning `sprintf(): Too few arguments` (JMail)

Arguments are assigned to RuntimeException constructor instead of sprintf

```HTML
<br />
<b>Warning</b>:  sprintf() [<a href='function.sprintf'>function.sprintf</a>]: Too few arguments in <b>*\libraries\joomla\mail\mail.php</b> on line <b>99</b><br />
<br />
<b>Fatal error</b>:  Wrong parameters for Exception([string $exception [, long $code [, Exception $previous = NULL]]]) in <b>*\libraries\joomla\mail\mail.php</b> on line <b>99</b><br />
```