Charm behaviour differs from grafana-agent for log_level config option

[MichaelThamm]

Enhancement Proposal

The log_level config option was introduced into grafana agent machine charm inspired by the functionality from the Prometheus charm.

• Add config option for log_level grafana-agent-operator#217

However, the above PR was implemented in a way that if an incorrect log level config was passed then the charm enters Blocked status rather than just silenty accepting the config and only creating a warning log message. The user may not realize their log level was not accepted.

Make this behaviour consistent here like in Grafana Agent.