An evidence update threshold setting that Monty does not define.

Mirrors ...evidence_matching/learning_module.py::InvalidEvidenceThresholdConfig, a ValueError raised when evidence_threshold_config is not one of the documented forms.

Here it is an exception used as the reason of an {:error, _} result, so an experiment with a mistyped threshold — "avg" instead of "mean", or "120%" — fails where the learning module is built, with a message that names the value, instead of producing a threshold that is quietly wrong.