[SPARK-4163][Core][WebUI] Send the fetch failure message back to Web UI

This is a PR to send the fetch failure message back to Web UI.
Before:
![f1](https://cloud.githubusercontent.com/assets/1000778/4856595/1f036c80-60be-11e4-956f-335147fbccb7.png)
![f2](https://cloud.githubusercontent.com/assets/1000778/4856596/1f11cbea-60be-11e4-8fe9-9f9b2b35c884.png)

After (Please ignore the meaning of exception, I threw it in the code directly because it's hard to simulate a fetch failure):
![e1](https://cloud.githubusercontent.com/assets/1000778/4856600/2657ea38-60be-11e4-9f2d-d56c5f900f10.png)
![e2](https://cloud.githubusercontent.com/assets/1000778/4856601/26595008-60be-11e4-912b-2744af786991.png)
Just to make sure we're on absolutely the same page, `private[storage]` is used to restrict visibility, to ensure that we never accidentally expose this API to a wider audience than necessary. `sealed` is used to restrict inheritance, so that the compiler (and programmer) can check that all cases have been matched on. The attributes are thus orthogonal in nature.

Adding `private[storage]` is good here, and we should also add it to the `trait FetchResult` as well, to make sure it's restricted in both visibility and inheritance.