Raise an error with better messages on non-registered observer used
Raise an error with better messages on non-registered observer used

jnishi/chainer,cupy/cupy,cupy/cupy,jnishi/chainer,kikusu/chainer,tkerola/chainer,pfnet/chainer,keisuke-umezawa/chainer,wkentaro/chainer,aonotas/chainer,okuta/chainer,hvy/chainer,anaruse/chainer,ktnyt/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,keisuke-umezawa/chainer,ktnyt/chainer,chainer/chainer,keisuke-umezawa/chainer,jnishi/chainer,wkentaro/chainer,ktnyt/chainer,keisuke-umezawa/chainer,niboshi/chainer,wkentaro/chainer,ktnyt/chainer,okuta/chainer,jnishi/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer,ysekky/chainer,kiyukuta/chainer,hvy/chainer,chainer/chainer,chainer/chainer,kashif/chainer,ronekko/chainer,okuta/chainer,niboshi/chainer,cupy/cupy,kikusu/chainer,cupy/cupy,okuta/chainer,delta2323/chainer,rezoo/chainer