Add some logging/notification when we can't send a transactional email

[floehopper]

In mailers where we call User#sending_suppressed? to decide whether or not to send an email, it would be good to know that we couldn't send the email we wanted. We probably don't care about broadcast emails (e.g. the newsletter) not being sent to a particular user, but I think it might be important to know about transactional emails we couldn't send.

[floehopper]

Also note @chrislo's comment from #142:

I think being a bit more granular about these suppressions would be an improvement too - if someone unsubscribes from the newsletter we may still want them to receive transactional emails, but maybe not if they've been supressed because their email bounces?