xap_helper: mset: ignore non-ultimate exceptions

xap_helper: mset: ignore non-ultimate exceptions

For requests consisting of multiple queries, ignore Xapian
exceptions for all but the last query.  When sending multiple
queries to the `mset' xap_helper command, we only care about the
result of the first successful query where success is defined as
a non-empty mset response.  Thus an exception is considered an
empty mset and we can try other queries from the same request.