improve error message of oras cp

[qweeah]

What happened in your environment?

Currently the error output of oras cp is confusing

oras cp --to-distribution-spec v1.1-referrers-tag $source_repo $target_repo
...
Error response from registry: sha256:0f58dd64e1e2b1f2c96e2b2cab30402540724a399061b035051534808114144a: not found	

What did you expect to happen?

The returned error should clarify

1. Where the error occurs in (which target, source or destination)
2. What operation causes the error.

Depends on oras-project/oras-go#677

How can we reproduce it?

Run oras cp with error returned during copying.

What is the version of your ORAS CLI?

v1.2.0

What is your OS environment?

Ubuntu 20.04

Are you willing to submit PRs to fix it?

• Yes, I am willing to fix it.