Implement "safeFields" option

See https://github.com/strongloop/loopback/issues/1650#issuecomment-161920555 for more details.

> ``` js
> // Array of fields (custom Error properties) that are safe to include
> // in response messages (both 4xx and 5xx).
> // "details" property is always included in 4xx responses (not in 5xx)
> safeFields: ['errorCode'],
> ```
