improve error message when user mistakenly uses tag list command

[qweeah]

Current behavior and output:

$ oras tag list ghcr.io/oras-project/oras
Error: unable to add tag for 'list': invalid reference: missing repository

Suggested error message:

$ oras tag list ghcr.io/oras-project/oras
Error: There is no "list" sub-command for "oras tag" command
Usage: oras tag [flags] <name>{:<tag>|@<digest>} <new_tag> [...]
If you want to list available tags in a repository, use "oras repo tags"