XFN Runner: Show STDOUT of failed composition function (exit code >= 1)

### What problem are you facing?

We've implemented a function similar to the example functions from [docs](https://docs.crossplane.io/master/concepts/composition-functions/#an-example-function). If this functions fails, it returns a exit code greater than 0 and prints the error message in STDOUT.

```
[..] "error": "cannot run Composition Function pipeline: cannot run function \"service-versioning\": cannot run container: rpc error: code = Unknown desc = exit status 1: xfn: error: spark.Command.Run(): OCI runtime error: exit status 1"
```

Currenty, I cannot debug a composition function that fails to execute. When the function in the OCI container fails, I see the exit code but no additional information printed in STDOUT.

### How could Crossplane help solve your problem?

Provide a way to see the STDOUT of functions that failed. That may be behind a flag/option like verbosity level/debug mode, etc.
Perhaps, also STDERR could be used, as it would be the more appropriate place for error messages.