fix(commands): classify command failures for retries

Allow command invocations to define which child exit codes are
considered successful, including checkupdates exit status 2 when no
updates are available.

Classify unexpected child command failures as exporter exit status 1 so
systemd treats them as runtime failures eligible for retry. Keep exit
status 2 for CLI usage errors, which are configuration problems and
should not be retried.

Update the systemd service with RestartPreventExitStatus=2 so only
retryable runtime failures trigger the configured restart policy.

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