Add warning when augmentations fail

[Aurelien-VB]

In the current code, when augmentations fail with attribute error, the error is silent and the code continues on.
In the best case it might not cause further problems but data will not be augmented.
In worst cases like for padding, the error may cause an other error down the line like in batch_list.

I propose to add warnings when such errors occurs so that developers can easily identify the cause of the problem.