Handle error event when listing revision data

Hello guys,

There's clearly something wrong with my setup - but I'm not exactly sure what *that* is - running `ember deploy --verbose env` yields the following:

```
+- fetchInitialRevisions
| |
| +- redis
- Listing initial revisions for key: `sunlight-web-client`
events.js:160
throw er; // Unhandled 'error' event

events.js:160
throw er; // Unhandled 'error' event
^

Error: Timed out while waiting for handshake
at Timeout._onTimeout (/Users/mariogintili/dev/sunlight/web-client/node_modules/ssh2/lib/client.js:569:19)
at ontimeout (timers.js:365:14)
at tryOnTimeout (timers.js:237:5)
at Timer.listOnTimeout (timers.js:207:5)
```

I was wondering if we could handle that error event to show something more informative?

**EDIT** the unhandled error might not be in this library, sorry if that's the case

首先重现 `ember deploy --verbose env`，并跟踪 `fetchInitialRevisions` 经过 `redis` 步骤。检查未处理的错误是来自此插件还是依赖项 `ssh2`；当相关故障已被识别，并产生一条信息明确的已处理错误时，即表示完成。

由索引模型根据 Issue 内容生成。