refuse to upgrade net with layer/layers inconsistency
refuse to upgrade net with layer/layers inconsistency

die loudly if a net definition (prototxt) mixes proto formats by
defining both `layer` and `layers` fields instead of complaining but
discarding and continuing.

fix #3381

tackgeun/caffe,gnina/gnina,tackgeun/caffe,gnina/gnina,CZCV/s-dilation-caffe,gnina/gnina,CZCV/s-dilation-caffe,tackgeun/caffe,gnina/gnina,gnina/gnina,gnina/gnina,CZCV/s-dilation-caffe,CZCV/s-dilation-caffe,tackgeun/caffe