Composable class returns `StreamingIP ` type for object that do not exist

[mariodruiz]

For instance,

ol = Overlay("cv_dfx_4_pr.bit")
cpipe = ol.composable

cpipe.noattribute

Returns

<pynq_composable.composable.StreamingIP at 0xffff61c1b5e0>

However, it should raise an AttributeError