Surface dropped attachments and addresses in the send log

An unreadable attachment or an invalid recipient was written to the
application log and nowhere else, so the send log reported a clean "sent"
for a mail that reached its recipient without the file they expected --
the exact case the send log exists to explain, and one that is close to
undiagnosable from the outside.

RenderedMessage now carries warnings, EmailGateway records them there as
well as in the log file, and SendLogListener writes them to the entry's
error column. The list shows that text on successful rows too, since a
message can be delivered and still be wrong.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
