Mail sender job should give meaningful error when it doesn't work due to permissions

When the mail sender scheduled job doesn't send anything due to a permissions problem (for example, "View all contacts" is not assigned to the sending user account), it should generate a meaningful error message in the job log. That should give the admin enough information to fix it. Something like, "Sending failed because user

does not have the

permission."

As it is now, the log says everything was successful, but no messages are ever sent.