Follow-up from "Feature extractors"

The following discussion from !112 (merged) should be addressed:

• @mguenther started a discussion: (+5 comments)

  I do not think that this is a good idea to have a default cropping for faces here, since all networks require a different cropping. Instead of using some obscure default cropping which might generate very bad features for the given network, I would rather recommend to raise an exception in case that we do not have eye locations.

  Note also that the dnn_default_cropping does not seem to be imported in this file.

Raise a warning during face-crop