MLBEDSW-3218: Added operator indices Quantize/Dequantize

Change-Id: Idcf1665f95ddecc2a12ff0e714f645263981d501
Signed-off-by: Louis Verhaard <louis.verhaard@arm.com>
diff --git a/ethosu/vela/pass_packing.py b/ethosu/vela/pass_packing.py
index 35e1b14..5673c2d 100644
--- a/ethosu/vela/pass_packing.py
+++ b/ethosu/vela/pass_packing.py
@@ -276,7 +276,7 @@
                             ):
                                 assert len(curr_op.inputs) >= 1
                                 ifm_tensor = curr_op.ifm
-                                assert ifm_tensor is not None
+                                assert ifm_tensor is not None, "IFM missing in {}".format(curr_op)
                                 assert ifm_tensor.purpose == TensorPurpose.FeatureMap
 
                         if flags_to_set & PassFlags.Dma: