ClientModule.java

// ...
requestStaticInjection(RestCallbackImpl.class);
bind(ConnectionErrorNotifier.class).to(ConnectionErrorConsoleNotifier.class).asEagerSingleton();
// ...

ConnectionErrorConsoleNotifier.java

public class ConnectionErrorConsoleNotifier implements ConnectionErrorNotifier{
    @Override
    public void onConnectionError(ConnectionErrorEvent event) {
        GWT.log("An error occured while communicating with the server");
    }

    @Inject
    public ConnectionErrorConsoleNotifier(EventBus eventBus) {
        eventBus.addHandler(ConnectionErrorEvent.TYPE, this);
    }
}

ConnectionErrorNotifier.java

public interface ConnectionErrorNotifier extends ConnectionErrorEventHandler {
}

RestCallbackImpl.java

public abstract class RestCallbackImpl<T> implements RestCallback<T>, HasHandlers {
    @Inject
    static EventBus eventBus;

    private Response response;

    @Override
    public void setResponse(Response response) {
        this.response = response;
    }

    @Override
    public void onFailure(Throwable throwable) {
        onError(response);
    }

    public void onError(Response response) {
        ConnectionErrorEvent.fire(this);
    }

    @Override
    public void fireEvent(GwtEvent<?> gwtEvent) {
        eventBus.fireEventFromSource(gwtEvent, this);
    }
}