notmuch_database_open: Fix error message for file-not-found.

notmuch_database_open: Fix error message for file-not-found.

I was incorrectly using the return value of stat (-1) instead of
errno (ENOENT) to try to construct the error message here.

Also, while we're here, reword the error message to not have
"stat" in it, which in spite of what a Unix programmer will
tell you, is not actually a word.