Avoid squashing errors during network startup cleanup path

Avoid squashing errors during network startup cleanup path

When failing to start a virtual network, we have to cleanup,
tearing down any iptables rules. If the iptables rules were
not present yet though, this raises an error, which squashes
the original error we were handling.

* src/network/bridge_driver.c: When failing to start a virtual
  network, don't squash the original error in cleanup