improve cudnn conv type error msg
improve cudnn conv type error msg

Summary: CuDNN conv op's type error was not very descriptive.

Reviewed By: Yangqing

Differential Revision: D5124638

fbshipit-source-id: 7d3f0afad36573cdb97d1f8ec3c60a9c6d87f926

pietern/caffe2,sf-wind/caffe2,pietern/caffe2,pietern/caffe2,pietern/caffe2,sf-wind/caffe2,davinwang/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,bwasti/caffe2,davinwang/caffe2,davinwang/caffe2,Yangqing/caffe2,bwasti/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,sf-wind/caffe2,caffe2/caffe2,sf-wind/caffe2,davinwang/caffe2,bwasti/caffe2,bwasti/caffe2,bwasti/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,xzturn/caffe2