refine error display

without addition props:

```
[Thu Nov 06 2014 11:51:17 GMT+0800 (CST)] ERROR 83839 [sdk-base] Unhandle SomeApiError: mock some error, 
Error Stack:
  Error: mock some error
    at null._onTimeout (/Users/deadhorse/git/sdk-base/test/index.test.js:56:19)
    at Timer.listOnTimeout (timers.js:133:15)
```

with addition props:

```
[Thu Nov 06 2014 11:51:17 GMT+0800 (CST)] ERROR 83839 [sdk-base] Unhandle SomeServiceClientError: mock error 1, 
Error Stack:
  Error: mock error 1
    at null._onTimeout (/Users/deadhorse/git/sdk-base/test/index.test.js:34:19)
    at Timer.listOnTimeout (timers.js:133:15)
Error Additions:
  data: {"foo":"bar","url":"/foo"}
  status: 500
  type: "DUMP"
```
what's this?