Agent should be more helpful when it isn't allowed to interact with systemd

[lfrancke]

Currently when the agent is not allowed to start/stop/etc. a service we'll get an error message like Interactive authentication required. (or similar depending on OS).

It'd be great if we could check for common causes here and suggest a fix in the log message

[siegfriedweber]

According to @lfrancke the error message "Interactive authentication required" was shown when the Stackable agent was started as non-root user but without the --session parameter.

In this case the current version of the agent fails with the following error messages:

[2021-08-27T09:23:30Z ERROR stackable_agent::provider::states::pod::creating_service] Failed to create systemd unit for service [default-agent-service-integration-test]
[2021-08-27T09:23:30Z ERROR krator::state] Object state machine exited with error. error=Read-only file system (os error 30)

This error message can be improved by showing the full path of the systemd unit file to make it clear that the agent does not try to create the file in the user's systemd directory.