[FR] Add juju error message when OS exporter throws error

### Description

There was an issue were the certificate was base64 encoded on accident using the juju config when the files should be raw. Doing a `systemctl status snap.charmed-openstack-exporter` indicated the error.

### Charm revision

31

### Juju version

3.6.4

### Cloud

OpenStack

### Expected behaviour

An error message in the juju status output. If clouds.yaml cannot be validated and there's an error in the systemctl status it should show an error in juju.

### Reproduce / Test

```
juju config openstack-exporter ssl_ca="$(base64 ca.cert)"
```

### Notes & References

_No response_