Fix error message for img_pool_layer

python/paddle/trainer_config_helpers/layers.py

@@ -2589,7 +2589,7 @@ def img_pool_layer(input,
 
     assert type(pool_type) in [AvgPooling, MaxPooling, CudnnAvgPooling,
                                CudnnMaxPooling], \
-        "only AvgPooling and MaxPooling are supported"
+        "only (Cudnn)AvgPooling, (Cudnn)MaxPooling are supported"
 
     if pool_type is None:
         pool_type = MaxPooling()