val result : ResultWithDiagnostics<EvaluationResult> = eval(...)
result.castOrError<ResultWithDiagnostics.Success<EvaluationResult>> {
 "script evaluation failed. $it"
}.value.returnValue.castOrError<ResultValue.Value> {
 "script evaluation result is $it"
}.value.castOrError<AppConfig> { // your expected class for value
 "script returns value, but it's not AppConfig. ${it?.javaClass?.simpleName}"
}.let { config = it }
inline fun <reified T:Any> Any?.castOrError(noinline messageCreator:(Any?)->String) =
 (this as? T) ?: error(messageCreator(this))