add assert when missing a reuseIdentifier

Summary: A cell without a reuseIdentifier will crash, but it'll be hard to debug which section-controller did it. Lets add an assert

Differential Revision: D56883578

fbshipit-source-id: 2f5a27d5f0a73f263eb58a14114edd607095cd4d

Source/IGListKit/Internal/IGListAdapter+UICollectionView.m

@@ -50,6 +50,9 @@ - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cell
     _isDequeuingCell = NO;
 
     IGAssert(cell != nil, @"Returned a nil cell at indexPath <%@> from section controller: <%@>", indexPath, sectionController);
+    if (cell) {
+        IGAssert(cell.reuseIdentifier != nil, @"Returned a cell without a reuseIdentifier at indexPath <%@> from section controller: <%@>", indexPath, sectionController);
+    }
 
     // associate the section controller with the cell so that we know which section controller is using it
     [self mapView:cell toSectionController:sectionController];