rpc: return errBrokenPipe on SendMessage in tests
rpc: return errBrokenPipe on SendMessage in tests

zombiezen/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto,hodduc/go-capnproto2,zombiezen/go-capnproto2,hodduc/go-capnproto2