[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.

I commented in the diff where these two fixes are added. The rest involves mainly style fixes.
I don't think this warning is a good idea. The worker also can become disassociated from the executor or driver actors, in those cases I'm not sure we want to log a warning.