Log the subject of mail errors in addition to the explanation

The subject often offers an indication of the action and email that
failed, while the explanation is usually pretty terse.  This is
especially true when the explanation is the direct result of an API
call.
Include Correspond/Comment in the subject when recording a mail error

This indicates for the RT admin looking in the logs what type of action
failed.
Merge branch '4.0/better-mail-error-logging' into 4.0-trunk