tools: Throw error if STARTTLS not supported

Throw error if STARTTLS is not supported on SMTP server when notifier
is configured to send emails using TLS (by default it falls back to
plain text).

Change-Id: Id1355ef6ac04dade33f3fff0294886dfaf84f0d6
Bug-Url: https://bugzilla.redhat.com/1062646
Signed-off-by: Martin Perina <mperina@redhat.com>