notmuch search: Fix to handle failure of notmuch_query_search_threads.

notmuch search: Fix to handle failure of notmuch_query_search_threads.

Now that notmuch_query_search_threads can return NULL, (for example,
due to a Xapian exception), we need to handle that case (rather than
just segfault). It's simple enough to just return a non-zero exit
code.