Log: debug user cancellation and log warning for rest

The context error with cancel code is typically for user cancellation which
should be at debug level. For other error codes we should display a warning.

Fixes etcd-io#9085

etcdserver/api/v3rpc/util.go

@@ -86,7 +86,11 @@ func isClientCtxErr(ctxErr error, err error) bool {
 	if ctxErr != nil {
 		return ctxErr == context.Canceled || ctxErr == context.DeadlineExceeded
 	}
-	ev, _ := status.FromError(err)
+
+	ev, ok := status.FromError(err)
+	if !ok {
+		return false
+	}
 	code := ev.Code()
 	return code == codes.Canceled || code == codes.DeadlineExceeded
 }