Throwing an exception from the blocker callback

[mglisse]

The move to cython3 unearthed the following issue: if we throw an exception from the python function used as callback in SimplexTree.expansion_with_blocker, it does not propagate nicely and instead we get a SystemError: "returned a result with an exception set". For now that's ok, throwing from this function is not that important, but it would be nice to investigate the possibilities there at some point.