launch: use error_origin() for external errors
launch: use error_origin() for external errors

When errors originate in an external utility, we must use error_origin()
over error_fold(), as the latter does not start new traces for fatal
errors.

Signed-off-by: David Herrmann <fb53b2ddb8d141e0cb39d7c67c2f81b6bc2eb0f7@gmail.com>

bus1/dbus-broker