xtask: is clueless about zombienet crashing

Sometimes when launching zombienet, node crashes. Here is the log:

```
Launching zombienet
assertion failed [block != nullptr]: BasicBlock requested for unrecognized address
(BuilderBase.h:550 block_for_offset)
 sh: line 1: 77538 Abort trap: 6           zombienet spawn -p native --dir zombienet testnet.toml
```

The problem is, xtask is never notified about this.