handleError now uses SilverStripe Error Handling

Since this is a Silverstripe module , i think, the default errorhandling from silverstripe should be used 
http://doc.silverstripe.org/framework/en/topics/error-handling

The ErrorLevel can easily be set with
'''
define('SMTPMAILER_SMTP_ERROR_LEVEL', E_USER_WARNING);
'''

This also gives the developer the opportunity to prevent the smtp-module from breaking the site only because an error with the mailer occurred