FIRQueryTests.mm: create failure message; needs to be updated to include the missing/unexpected document IDs

Author: dconeybe

Firestore/Example/Tests/Integration/API/FIRQueryTests.mm

@@ -1271,8 +1271,7 @@ - (void)testResumingAQueryShouldUseExistenceFilterToDetectDeletes {
     if (! [actualDocumentIds isEqualToSet:expectedDocumentIds]) {
       NSArray<NSString*>* unexpectedDocumentIds = SortedStringsNotIn(actualDocumentIds, expectedDocumentIds);
       NSArray<NSString*>* missingDocumentIds = SortedStringsNotIn(expectedDocumentIds, actualDocumentIds);
-      (void)unexpectedDocumentIds;
-      (void)missingDocumentIds;
+      XCTFail(@"The snapshot contained %@ documents (expected %@), %@ unexpected and %@ missing", @(actualDocumentIds.count), @(expectedDocumentIds.count), @(unexpectedDocumentIds.count), @(missingDocumentIds.count));
     }
   }
 }