Give more details in bulk email sending if a user has no email defined

[silviot]

When sending a bulk email the system warns about missing addresses.
It would be nice if it listed the names of the athletes who won't receive the email. Also we could improve on the clarity of the two buttons. It is my understanding that the left one will abort the operation, while the right one will send the emails when possible, but the fact I'm not 100% sure is a symptom it can be improved.