We should make Puma raise VentureExceptions

[axch]

Right now, Puma is coded to just throw some string whenever anything bad happens. In Python, this comes out as a RuntimeError, even though in many cases a VentureException would be considerably more helpful, for instance because it would then be subject to at least minimal location annotation.

Doing this for the old CXX backend was my starter project when I joined the lab years ago, and I didn't succeed then; but perhaps now it's not too difficult?