Gatherer "systemd" and "package_version" commandline help improvement

[pirat013]

In case the "--argument" option is forgotten, the error message doesn't give a good hint what going wrong:

INFO[2022-12-15 10:41:57] Using config file: /etc/trento/agent.yaml    
INFO[2022-12-15 10:41:57] loading plugins                              
INFO[2022-12-15 10:41:57] Starting systemd state facts gathering process 
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/trento-project/agent/internal/factsengine/gatherers.(*SystemDGatherer).Gather(0xc00000d770, {0xc0001ca640, 0x1, 0x7?})
	github.com/trento-project/agent/internal/factsengine/gatherers/systemd.go:81 +0x6ad
github.com/trento-project/agent/cmd.gather(0xc00024c900, {0xbce614, 0x3, 0x3})
	github.com/trento-project/agent/cmd/facts.go:114 +0x403
github.com/spf13/cobra.(*Command).execute(0xc00024c900, {0xc000200c90, 0x3, 0x3})
	github.com/spf13/cobra@v1.6.1/command.go:920 +0x847
github.com/spf13/cobra.(*Command).ExecuteC(0xc00024c000)
	github.com/spf13/cobra@v1.6.1/command.go:1044 +0x3bc
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v1.6.1/command.go:968
github.com/trento-project/agent/cmd.Execute()
	github.com/trento-project/agent/cmd/root.go:16 +0x1e
main.main()
	github.com/trento-project/agent/main.go:8 +0x17

An error mentioning that the "--argument" option is missing would be much more helpful.

[nelsonkopliku]

Thanks @pirat013. Tracked.

[nelsonkopliku]

Addition: to me it's not happening for the package_version gatherer.

./trento-agent facts gather --gatherer "package_version"
INFO[2022-12-15 12:43:09] {
  "Name": "",
  "CheckID": "",
  "Value": null,
  "Error": {
    "Message": "error getting version of package: ",
    "Type": "package-version-cmd-error"
  }
}

[pirat013]

Yes, you are right. I mixed them up. But a hint that the --argument is missing would be nice.

[nelsonkopliku]

To follow the PR about this discussion trento-project/agent#162

EDIT: merged