[SPARK-7502] DAG visualization: gracefully handle removed stages

Old stages are removed without much feedback to the user. This happens very often in streaming. See screenshots below for more detail. zsxwing

**Before**

<img src="https://cloud.githubusercontent.com/assets/2133137/7621031/643cc1e0-f978-11e4-8f42-09decaac44a7.png" width="500px"/>

-------------------------
**After**
<img src="https://cloud.githubusercontent.com/assets/2133137/7621037/6e37348c-f978-11e4-84a5-e44e154f9b13.png" width="400px"/>

Author: Andrew Or <andrew@databricks.com>

Closes #6132 from andrewor14/dag-viz-remove-gracefully and squashes the following commits:

43175cd [Andrew Or] Handle removed jobs and stages gracefully