Clean up network on failure

Remove veth and/or netns symlink if interface configuration fails part-way through.
One common cause of such failures is that the container itself has exited, as described in #61.

The change consists of using either 'if' or '&&' to avoid 'set -e' terminating the script on first failure, then carrying the real success/failure status in EXITCODE.

Once we have managed to attach the veth interface to the container's netns, it will go away when the container exits, so at that point we don't need to clean it up ourselves.