Improve get_model rpc error handling

`get_model` does not handle connection errors, so it does not contain server/port actually failed, which prohibits the logging policy.

Before applying this patch:

```
I0514 17:04:40.945291  1258 linear_mixer.cpp:413] start to get model from other server
W0514 17:04:49.955059  1258 linear_mixer.cpp:425] stabilizer exception: connect failed
```

After applying this patch:

```
I0514 18:16:37.019196  5300 linear_mixer.cpp:399] start to get model from other server
W0514 18:16:46.031011  5300 linear_mixer.cpp:164] get_model failed (connect failed): 192.168.122.211:9199
W0514 18:16:46.031390  5300 linear_mixer.cpp:411] stabilizer exception: connect failed
```