Function value_is_error_only 

pub fn value_is_error_only(value: &Value) -> bool

True when value is a Value::Error or a non-empty list of only errors.

Commands like str length embed type mismatches as values via operate() instead of failing the pipeline. Interactive last-result must not replace $ans.last with those payloads (same policy as a thrown runtime error). Mixed lists (some errors, some values) are real results and return false.