improve error message when no tag or digest provided

[qweeah]

Current behavior and output:

$ oras manifest fetch --oci-layout /tmp/ginkgo1163328512
Error: "/tmp/ginkgo1163328512": no tag or digest when expecting <name:tag|name@digest>

Suggested error message:

$ oras manifest fetch --oci-layout /tmp/ginkgo1163328512
Error: "/tmp/ginkgo1163328512": no tag or digest specified
Usage: oras manifest fetch [flags] <name>{:<tag>|@<digest>}
You need to specify an artifact reference in the form of "<name>:<tag>" or "<name>@<digest>". Run "oras manifest fetch -h" for more options and examples 

[wangxiaoxuan273]

I would like to work on this issue.

[qweeah]

This change can also be applied to

• oras attach
• oras cp
• oras discover
• oras pull
• oras resolve
• oras delete
• oras tag
• oras manifest delete
• oras blob delete (please be aware that there is not tag in the hint)
• oras manifest fetch-config