Make error messages more informative 

# Example of error message to improve

```text
(e.g. this appears when a user has been already added to Slurm Failed to create Slurm user. Command '/usr/local/bin/sacctmgr add user...
```

anyhow as a crate could help. See [issue](https://github.com/th-nuernberg/usermgmt/issues/10)