Fix "notmuch new" (bad performance, and no committing of results).

Fix "notmuch new" (bad performance, and no committing of results).

We were incorrectly only destroying messages in the case of
successful addition to the database, and not in other cases,
(such as failure due to FILE_NOT_EMAIL).

I'm still not entirely sure why this was performing abysmally, (as in
making an operation that should take a small fraction of a second take
10 seconds), nor why it was causing the database to entirely fail to
get new results.

But fortunately, this all seems to work now.