[SPARK-2258 / 2266] Fix a few worker UI bugs

**SPARK-2258.** Worker UI displays zombie processes if the executor throws an exception before a process is launched. This is because we only inform the Worker of the change if the process is already launched, which in this case it isn't.

**SPARK-2266.** We expose "Some(app-id)" on the log page. This is fairly minor.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1213 from andrewor14/fix-worker-ui and squashes the following commits:

c1223fe [Andrew Or] Fix worker UI bugs