[train v2] Improve TrainingFailedError message

[justinvyu]

Summary

The previous error message tells the user to look for error logs elsewhere, and it also references a worker_failures attribute on the error object itself, which is inaccessible to users who did not try catch the call to trainer.fit(). This PR improves the error message by just including the error string so that it's easy to see exactly what happened on the workers at a glance.

Here's what the message looks like now:

Training failed due to worker errors:
[Rank 0]
Traceback (most recent call last):
  File "/Users/justin/Developer/ray/python/ray/train/v2/tests/test_data_parallel_trainer.py", line 145, in _error_func_rank_0
    raise ValueError("error")
ValueError: error

Follow-up work

• If there are a lot of workers that failed at once, this error message could get really long. This is the same for the controller warnings that include the same error string.

[justinvyu]

Should this be Public(beta)?