Add NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID

Add NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID

And document that notmuch_database_add_message can return this
value. This pushes the hard decision of what to do with duplicate
messages out to the user, but that's OK. (We weren't really doing
anything with these ourselves, and this way the user is at least
informed of the issue, rather than it just getting papered over
internally.)