fix: Improve namespace validation logic for update, inspect command

[opjt]

related issue #3851
related PR #4618

changes

namespace update

1. Added validation to require the --label flag for the update command.

pjt@lima-default:/Users/pjt/Projects/PJT/fork-git/nerdctl$ nerdctl namespace update 1
FATA[0000] required flag(s) "label" not set

2. Added namespace existence check logic

pjt@lima-default:/Users/pjt/Projects/PJT/fork-git/nerdctl$ nerdctl namespace update 11 --label=1
FATA[0000] namespace 11: not found

namespace inspect

1. Also added namespace existence check logic for inspect

pjt@lima-default:/Users/pjt/Projects/PJT/fork-git/nerdctl$ nerdctl namespace inspect 1 11
[
    {
        "Name": "1",
        "Labels": {
            "1": "true"
        }
    }
]

WARN[0000] namespace 11: not found

[AkihiroSuda]

Thanks