Forward exceptions from twinterpreters

[maxfischer2781]

Exceptions happening in twinterpreters are inconsistently forwarded, sometimes not at all. All user-space exceptions should be forwarded, an possibly provide introspection.

[maxfischer2781]

Exceptions occurring when receiving a request are not handled by the RequestHandler: server._server_recv is called outside its scope. Any exceptions for unpickling, e.g. missing dependencies, cause the server to crash.

[maxfischer2781]

Global, unhandled exceptions cause a graceful shutdown of twins and a regular traceback since 9e7c867.

[maxfischer2781]

Python3 has the multiprocessing.pool.RemoteTraceback, which is probably worth looking into.