Highlight exceptions in Task Groups visualization

If a task errs we would like for this to be visible in the Task Group visualization. I suggest that we make the background some dark-ish shade of black.

We might also include the exception and possibly traceback directly in the hover. To do this we would need to send the string representation of the exception and traceback from the Worker to the Scheduler. Currently I think we send pickled versions, which we shouldn't unpack on the scheduler. We would probably need to send both to serve both needs.

Currently we store the exception/traceback on the worker.py:TaskState object, but in pickled form (see distributed.core.error_message). Maybe we should store the non-pickled version, and then send both the pickled and stringified versions up in `Worker.send_task_state_to_scheduler` method.