Convert exception to string in driver

[danielmitterdorfer]

With this commit we convert exceptions that occur in a load generator
actor before we communicate a BenchmarkFailure to the Driver actor.
This avoids deserialization errors in case the exception type is defined
in a track-specific extension.

Closes #498

[danielmitterdorfer]

Here is an example from the original reproduction scenario in the ticket. In rally-actor-messages.log we see:

2018-05-14 11:10:05,625 ActorAddr-(T|:53524)/PID:10170 rally.driver ERROR Could not execute schedule
Traceback (most recent call last):

  File "/Users/daniel/Projects/rally/esrally/driver/driver.py", line 998, in __call__
    total_ops, total_ops_unit, request_meta_data = execute_single(runner, self.es, params, self.abort_on_error)

  File "/Users/daniel/Projects/rally/esrally/driver/driver.py", line 1030, in execute_single
    return_value = runner(es, params)

  File "/Users/daniel/Projects/rally/esrally/driver/runner.py", line 80, in __call__
    return self.runnable(es['default'], *args[1:])

  File "/Users/daniel/Downloads/rally-eventdata-track/eventdata/runners/fieldstats_runner.py", line 41, in fieldstats
    raise ParameterError("No matching data found for field '{}' in pattern '{}'.".format(params['fieldname'], params['index_pattern']));

Note that ParameterError is defined by the track. In the actor system's internal log file we see:

2018-05-14 13:10:10.625113 p10160 I    OUCH!  Error deserializing received data: Traceback (most recent call last):
  File "/Users/daniel/Projects/rally/.venv/lib/python3.6/site-packages/thespian-3.8.3-py3.6.egg/thespian/system/transport/TCPTransport.py", line 1431, in _addedDataToIncoming
    rdata, extra = inc.data
  File "/Users/daniel/Projects/rally/.venv/lib/python3.6/site-packages/thespian-3.8.3-py3.6.egg/thespian/system/transport/TCPTransport.py", line 185, in data
    def data(self): return self._rData.completed()
  File "/Users/daniel/Projects/rally/.venv/lib/python3.6/site-packages/thespian-3.8.3-py3.6.egg/thespian/system/transport/streamBuffer.py", line 80, in completed
    return self._deserialize(self._buf), self._extra
ModuleNotFoundError: No module named 'eventdata'
  (rdata="", extra="")

[dliappis]

Thanks for catching this very subtle bug! LGTM

[danielmitterdorfer]

Thanks for the quick review!