Print email addresses after errors as well

While the basic problem here is that drupal_mail() can't give any more useful information than php's mail(), it would help to have some more feedback after a problem than simply "X errors encountered sending message. Please check the logs and try again." In particular, if run on a shared host without access to mail server log files, it can be hard to figure out what happened, so it would be nice to be given as much feedback as possible. Sometimes one batch of addresses might be sent successfully and other not, so it would be helpful to know if that happened. In addition, the current error message implies that the watchdog log as a record of problems with the mail delivery, and that's not the case.

I don't think this is needlessly verbose, since it doesn't print any more text than it would if the mail delivery succeeded.