Instead of a single search, a sequence of searches are now run

sequentially until we get one that finds one or more records.  This
enables us to test for records in, for example, indexdata.com/marc,
which previously got zero hits and then gave up.  So far, there is
only one fallback query, but more can be added trivially.

Also arranged for the last retrieval callback to return TEST_GOOD
rather than TASK_DONE, which makes no real difference but is more
polite.