Improve error message

[siegfriedweber]

Description

Error message improved which is logged if a systemd unit file cannot be created.

The log output looks like this:

[2021-08-27T13:14:26Z ERROR krator::state] Object state machine exited with error. error=Unit file [default-agent-service-integration-test.service] could not be created

    Caused by:
        0: File [/lib/systemd/system/default-agent-service-integration-test.service] could not be created
        1: Read-only file system (os error 30)

Fixes #47

Checklist

• Code contains useful comments
• (Integration-)Test cases added (or not applicable)
• Documentation added (or not applicable)
• Changelog updated (or not applicable)

[maltesander]

LGTM.