grpcproxy: fix "grpc.ErrClientConnClosing" handling
grpcproxy: fix "grpc.ErrClientConnClosing" handling

Fix

```
go test -v -tags cluster_proxy -run TestWatchErrConnClosed
```

with gRPC >= v1.10

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

xiang90/etcd,bdarnell/etcd,rnaveiras/etcd,vimalk78/etcd,bdarnell/etcd,mqliang/etcd,dannysauer/etcd,xiang90/etcd,mkumatag/etcd,etcd-io/etcd,etcd-io/etcd,ajityagaty/etcd,mqliang/etcd,vimalk78/etcd,dannysauer/etcd,mkumatag/etcd,gyuho/etcd,joshix/etcd,joshix/etcd,sinsharat/etcd,coreos/etcd,mkumatag/etcd,sinsharat/etcd,mqliang/etcd,li-ang/etcd,ajityagaty/etcd,cockroachdb/etcd,rnaveiras/etcd,gyuho/etcd,rnaveiras/etcd,li-ang/etcd,etcd-io/etcd,ajityagaty/etcd,coreos/etcd,cockroachdb/etcd