Improve feedback on errors

[danielmitterdorfer]

With Rally 0.4.0 we decoupled the internal load driver. This leads to situations where users get feedback like this:

Traceback (most recent call last):
File "/home/ubuntu/.local/lib/python3.5/site-packages/esrally/racecontrol.py", line 364, in run
pipeline()
File "/home/ubuntu/.local/lib/python3.5/site-packages/esrally/racecontrol.py", line 60, in call
step()
File "/home/ubuntu/.local/lib/python3.5/site-packages/esrally/racecontrol.py", line 29, in call
self.command(self.ctx)
File "/home/ubuntu/.local/lib/python3.5/site-packages/esrally/racecontrol.py", line 145, in benchmark_internal
raise exceptions.RallyError("Driver has returned no metrics but instead [%s]. Terminating race without result." % str(completed))
esrally.exceptions.RallyError: Driver has returned no metrics but instead [Poison<>]. Terminating race without result.

We should add an ability to communicate errors to the master process more clearly and provide users a better feedback message than just a trace.