Fix Bug when trying to send a mail after the SMTPMailSender threw an exception

Added a abort() function to cancel the current process.
In case the SMTPMailSender throws an exception which is catched by another script, no further mails could be sent because the last command was not aborted.
It may be a good idea to use a different exception type than "SystemException"
Merge pull request #1586 from schlimpf/patch-2

Fix Bug when trying to send a mail after the SMTPMailSender threw