dma: treat a QUIT error as merely a warning

dma: treat a QUIT error as merely a warning

RFC 2821 only mandates that a QUIT error should abort an unfinished
transaction, and since we've reached this point, the DATA command has
succeeded and the message has been accepted for delivery by the remote
end.  Thus, just warn about it.

Submitted-by: Peter Pentchev <roam@ringlet.net>