Warn on request errors

[danielmitterdorfer]

By default Rally continues on request errors (--on-error=continue) but this might be easy to miss by users. In addition, some operations are treated as "administrative" (e.g. create-index) and are thus not included in the command line report by default. We should:

1. Include any operations with an error rate greater than zero in the command line report regardless whether they should be included (see

   rally/esrally/metrics.py

   Line 1631 in 97d0e85

   if task.operation.include_in_reporting:

   )
2. Emit a warning when the error rate of an operation is greater than zero (in

   rally/esrally/reporter.py

   Line 134 in 97d0e85

   def add_warnings(self, warnings, values, op):

   )

[bartier]

@danielmitterdorfer Hi! Can I work on this?

1. I'd purpose something in the lines:
   if task.operation.include_in_reporting or error_rate > 0
   "Administrative" operations would be in the report if error happens (error_rate > 0)

2. Add warnings at the end of the command line report:

def add_warnings(self, warnings, values, op):
    if values["error_rate"] > 0:
        warnings.append(f"Error rate is {values['error_rate']} for operation '{op}'. Please check the logs.")
   ...
   ...

[danielmitterdorfer]

@bartier sure, thanks for working on it. :)

I thought about your idea to force-include failed tasks in the report as follows:

 if task.operation.include_in_reporting or error_rate > 0 

and I think that makes sense. Looking forward to your PR.