public SELF onUnprocessed() {
        return onException(UnprocessedRequestException.class);
    }
Adds an {@link UnprocessedRequestException}.

<p>When this is used for a {@link CircuitBreakerRule}, a {@link Response} is reported as a success or
failure to a {@link CircuitBreaker} or ignored depending on the build methods({@code #thenSuccess()},
{@code #thenFailure()} and {@code #thenIgnore()}).
core/src/main/java/com/linecorp/armeria/client/AbstractRuleBuilder.java