improve the returned error of `oci.NewFromTar` when the passed-in path is not a tarfile

[Billy Zha]

oci.NewFromTar can be used to create a new read-only OCI store from a tarfile path. If the passed-in path doesn't point to a tarfile, currently(v2.3.1) below errors will be returned:

• If the length of the file is zero, a failed to open OCI layout file: oci-layout: file does not exist error will be returned
• If the length of the file is not zero, an io.ErrUnexpectedEOF will be returned

As a user of oras-go SDK, I would like this function to return a unified and exported error type. Also, the error should indicate that the passed-in path doesn't point to an expected tarfile.

[Adnan Gulegulzar]

Hello

I would like to work on this issue.

I have tried implementing the required code changes in the PR #793 as best I could interpret them.
Please let me know if any changes are required.

[Shiwei Zhang]

@ADorigi Thanks for your contribution. After discussing with @qweeah offline, this issue is no longer valid.

The more user-friendly error messages should be handled in the CLI instead of in the oras-go SDK.

A proper example of oci.NewFromTar error handling would be

store, err := oci.NewFromTar(ctx, path)
if err != nil {
  return fmt.Errorf("%s: invalid or corrupted oci-style tarball: %w", path, err)
}

[Billy Zha]

Thanks @ADorigi for the contribution. io.ErrUnexpectedEOF already implies corrupted(invalid) tar ball file.

Also, we notice that failed to open OCI layout file: oci-layout: file does not exist got an extra space. It's caused by missing operation in the path error. Will raise a PR to fix that.