don't allow having a cache without backend, as fallback use BlackHole backend
don't allow having a cache without backend, as fallback use BlackHole backend

fixes error during exception handling when cache/simple is not writeable

koala-framework/koala-framework,yacon/koala-framework,Ben-Ho/koala-framework,kaufmo/koala-framework,darimpulso/koala-framework,kaufmo/koala-framework,Sogl/koala-framework,nsams/koala-framework,Ben-Ho/koala-framework,darimpulso/koala-framework,yacon/koala-framework,nsams/koala-framework,nsams/koala-framework,Sogl/koala-framework,kaufmo/koala-framework,koala-framework/koala-framework,yacon/koala-framework