Cast our unknown error to an Error

useAsync generally does not know how what its tasks are going to be
rejected with, hence the unknown.

For these API calls we know that it will be an Error, but I don't
currently have a way to type that generally. For now, we'll cast it
where we use it.

x-pack/plugins/security_solution/public/detections/components/value_lists_management_modal/form.tsx

@@ -107,7 +107,7 @@ export const ValueListsFormComponent: React.FC<ValueListsFormProps> = ({ onError
     if (!importState.loading && importState.result) {
       handleSuccess(importState.result);
     } else if (!importState.loading && importState.error) {
-      handleError(importState.error);
+      handleError(importState.error as Error);
     }
   }, [handleError, handleSuccess, importState.error, importState.loading, importState.result]);