Raise an error when cuda import failed.
Raise an error when cuda import failed.

In a special situation, package is found though import fails.
Thanks to @jheymann85

chainer/chainer,woodshop/chainer,ytoyama/yans_chainer_hackathon,sou81821/chainer,cupy/cupy,tkerola/chainer,niboshi/chainer,wkentaro/chainer,AlpacaDB/chainer,ktnyt/chainer,t-abe/chainer,Kaisuke5/chainer,ysekky/chainer,umitanuki/chainer,jnishi/chainer,ikasumi/chainer,jnishi/chainer,rezoo/chainer,keisuke-umezawa/chainer,kikusu/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,cupy/cupy,niboshi/chainer,okuta/chainer,1986ks/chainer,kikusu/chainer,muupan/chainer,hidenori-t/chainer,keisuke-umezawa/chainer,truongdq/chainer,kuwa32/chainer,chainer/chainer,tigerneil/chainer,wavelets/chainer,niboshi/chainer,t-abe/chainer,cupy/cupy,hvy/chainer,benob/chainer,woodshop/complex-chainer,anaruse/chainer,wkentaro/chainer,wkentaro/chainer,okuta/chainer,hvy/chainer,sinhrks/chainer,kashif/chainer,kiyukuta/chainer,ktnyt/chainer,cemoody/chainer,muupan/chainer,cupy/cupy,jnishi/chainer,pfnet/chainer,aonotas/chainer,minhpqn/chainer,masia02/chainer,yanweifu/chainer,jnishi/chainer,ronekko/chainer,truongdq/chainer,chainer/chainer,ktnyt/chainer,laysakura/chainer,hvy/chainer,keisuke-umezawa/chainer,AlpacaDB/chainer,sinhrks/chainer,delta2323/chainer,ktnyt/chainer,okuta/chainer,chainer/chainer,wkentaro/chainer,benob/chainer,keisuke-umezawa/chainer,tscohen/chainer