Errors with EvolutionStrategy in MOO benchmarks

## Steps to reproduce

1. run benchmarks by using command python -m nevergrad.benchmark multiobjective_example_many —num_workers=76 —plot

## Observed Results
When running the the benchmarks to check performance of the optimizers in a MOO context I noticed these errors from the EvolutionStrategies optimizer in particular. The benchmarks aren't running for some of the optimizers and this is the error that is given. I ran the same command on an older version of master to ensure the no_hypervolume changes didn’t introduce these errors and saw that the same errors did crop up even before those changes.
```
Starting 282 (4/16 of worker): Experiment: Experiment: EvolutionStrategy(offsprings=200, recombination_ratio=0.5) (dim=7, param=:[0. 0. 0. 0. 0. 0. 0.]) on Instance of MultiExperiment(function_class='MultiExperiment', name='sphere,sphere,sphere,sphere') with seed None
Error when applying Experiment: Experiment: EvolutionStrategy(offsprings=200, recombination_ratio=0.5) (dim=7, param=:[0. 0. 0. 0. 0. 0. 0.]) on Instance of MultiExperiment(function_class='MultiExperiment', name='sphere,sphere,sphere,sphere') with seed None:
Traceback (most recent call last):
File "/private/home/theoajclarke/nevergrad/nevergrad/benchmark/xpbase.py", line 196, in run
self._run_with_error()
File "/private/home/theoajclarke/nevergrad/nevergrad/benchmark/xpbase.py", line 274, in _run_with_error
raise e
File "/private/home/theoajclarke/nevergrad/nevergrad/benchmark/xpbase.py", line 266, in _run_with_error
obase.Optimizer.minimize(
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/base.py", line 641, in minimize
args = self.ask()
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/base.py", line 462, in ask
candidate = self._internal_ask_candidate()
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/es.py", line 53, in _internal_ask_candidate
uid = self._uid_queue.ask()
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/utils.py", line 329, in ask
raise RuntimeError("Both asked and told queues are empty.")
RuntimeError: Both asked and told queues are empty.
```
```
Starting 319 (5/16 of worker): Experiment: Experiment: RecMixES (dim=6, param=:[0. 0. 0. 0. 0. 0.]) on Instance of MultiExperiment(function_class='MultiExperiment', name='sphere,sphere,sphere,sphere,sphere,sphere') with seed None
Error when applying Experiment: Experiment: RecMixES (dim=6, param=:[0. 0. 0. 0. 0. 0.]) on Instance of MultiExperiment(function_class='MultiExperiment', name='sphere,sphere,sphere,sphere,sphere,sphere') with seed None:
Traceback (most recent call last):
File "/private/home/theoajclarke/nevergrad/nevergrad/benchmark/xpbase.py", line 196, in run
self._run_with_error()
File "/private/home/theoajclarke/nevergrad/nevergrad/benchmark/xpbase.py", line 274, in _run_with_error
raise e
File "/private/home/theoajclarke/nevergrad/nevergrad/benchmark/xpbase.py", line 266, in _run_with_error
obase.Optimizer.minimize(
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/base.py", line 624, in minimize
self.tell(x, result)
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/base.py", line 376, in tell
self._internal_tell_candidate(candidate, loss)
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/es.py", line 77, in _internal_tell_candidate
self._select()
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/es.py", line 82, in _select
choices_rank = self._rank_method(choices, n_selected=self._config.popsize)
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/multiobjective/nsga2.py", line 212, in rank
frontiers = frontier_ranker.compute_ranking(population)
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/multiobjective/nsga2.py", line 162, in compute_ranking
dominance_test_result = self.compare(uid2candidate[uid1], uid2candidate[uid2])
File "/private/home/theoajclarke/nevergrad/nevergrad/optimization/multiobjective/nsga2.py", line 129, in compare
one_wins = np.sum(candidate1.losses < candidate2.losses)
File "/private/home/theoajclarke/nevergrad/nevergrad/parametrization/core.py", line 82, in losses
raise RuntimeError("No loss was provided")
RuntimeError: No loss was provided
```

## Expected Results

I would expect the benchmarks to run normally and not produce errors.

## Relevant Code