The master client print too many logs when `GetTask` error.

[gongweibao]

The master client print too many logs when GetTask error. It's a dead loop.

t=2017-10-25T09:38:18+0000 lvl=eror msg="getTask error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:138]
t=2017-10-25T09:38:18+0000 lvl=eror msg="task finish callback error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:172]
t=2017-10-25T09:38:18+0000 lvl=eror msg="getTask error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:138]
t=2017-10-25T09:38:18+0000 lvl=eror msg="task finish callback error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:172]
t=2017-10-25T09:38:18+0000 lvl=eror msg="getTask error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:138]
t=2017-10-25T09:38:18+0000 lvl=eror msg="task finish callback error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:172]
t=2017-10-25T09:38:18+0000 lvl=eror msg="getTask error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:138]
t=2017-10-25T09:38:18+0000 lvl=eror msg="task finish callback error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:172]
t=2017-10-25T09:38:18+0000 lvl=eror msg="getTask error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:138]
t=2017-10-25T09:38:18+0000 lvl=eror msg="task finish callback error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:172]
t=2017-10-25T09:38:18+0000 lvl=eror msg="getTask error." error="connection is shut down" stack=[github.com/PaddlePaddle/Paddle/go/master/client.go:138]