@Test
    void testNonexistingBackendCausesError() {
        BlendingSearcherWrapper searcher = setupFirstAndSecond();
        Query query = new Query("/search?query=banana&search=nonesuch");

        Result result = new Execution(searcher, Execution.Context.createContextStub()).search(query);
        assertEquals(0, result.getConcreteHitCount());
        assertNotNull(result.hits().getError());
        ErrorMessage e = result.hits().getError();
        assertEquals("Invalid query parameter", e.getMessage());
        //assertEquals("No source named 'nonesuch' to search. Valid sources are [first, second]",
        //             e.getDetailedMessage());
    }
Disabled because the document source searcher does not handle being asked for
document sumaries for hits it did not create (it will insert the wrong values).
But are we sure fsearch handles this case correctly?
container-search/src/test/java/com/yahoo/prelude/searcher/test/BlendingSearcherTestCase.java