[enhancement]: runcmd failure debugging

# Enhancement

When a runcmd fails, cloud-init provides a warning log:

```
Failed to run module...
```

This leaves the user with an indication that something went wrong, but fails to address the problem that the user now gets to solve "what went wrong and how do I fix it?".

Providing easier introspection would be a huge benefit for this frequently-used module.

Currently stdout/stderr go to `/var/log/cloud-init-output.log`. We should at the very least document this in the module docs.