[kbn-evals] Experiment aborts on any exception

### Summary

The executor has no error handling around evaluator calls. One network blip during a long run kills everything. Already-ingested scores survive, but the rest of the run is lost with no partial result.

### Problem

`executor/client.py:174-180` calls `await evaluator.evaluate(params)` with no try/except. `asyncio.gather` without `return_exceptions=True` means one failure propagates and aborts the whole run.

Kibana evaluators propagate exceptions. The executor has no safety net.

### Fix

Wrap evaluator calls in the executor. Catch exceptions, convert to `EvaluationResult(label="error", explanation=...)`, log at error level, and continue the run.

kbn-evals wraps every evaluator call in try/catch and converts exceptions to a `{status: "error"}` entry without aborting the run. The SDK should mirror that.

### Done when

- [ ] A failed evaluator does not abort other evaluators or the run
- [ ] The failed score is ingested with `label="error"` and an explanation
- [ ] The test `test_kibana_evaluators_propagate_request_errors` is updated to match the new policy

Start in executor/client.py:174-180, where evaluator.evaluate(params) is called, and run test_kibana_evaluators_propagate_request_errors. Trace the asyncio.gather path and existing EvaluationResult handling. Done means evaluator failures are logged, ingested as label="error" with an explanation, and do not abort other evaluators or the run.

Written by the indexing model from the issue text.