[Debugger] DebugReportGenerator needs some additional error handling

[anevilyak]

It's possible for the target team to go away while we're in the middle of generating a report, if one of the target team's threads is still active and issues an exit() or other termination call. In such a case, the report generator will currently have a high chance of hanging if it happens to be in the middle of waiting for some resource such as a stack trace to be retrieved. We need to watch for the corresponding events / worker failures and wake up/abort accordingly.

[luroh]

Moving Debugger related tickets out of R1 milestone (Prop #14).