Fix a bug in removing executors in StandaloneSchedulerBackend

Also improve logging when the executor being removed is not found.

Observed by debugging: executor names are passed in to `removeExecutor` as "appId/executorId" strings but the keys of the `executorActor` map are just executor IDs without app IDs. The effect of this is that executors are not properly removed from a running application, and the application becomes very unresponsive even when a single executor crashes.

@markhamstra