Clear the emailed flag after we actually have sent the email

Looking at the node_down subscription it seems you are clearing the flag to send an email before you actually sent it:

                    # Relay has been down for more than the waiting time
                    if not self.databaseSubscription.emailed:
                        # Subscriber was not sent an email already
                        self.databaseSubscription.emailed = True
                        if self.databaseSubscription.is_active:
                            # The subscription is active, we should send an email
                            self.sendEmail()

But let's assume there is an error occurring in the sendEmail() part that actually prevents the email from getting sent out. I think in that case we should not clear out the emailed flag. Rather, what I assume happening in that case is that we a) might want to log an error and b) re-try sending that mail during the next run, and if successful then set emailed to True.