oci.Storage.Delete() should return ErrNotFound if target is not present in the storage

[Wwwsylvia]

In the current implementation, a *PathError will be return if target is not present in the storage, which is not user-friendly.

oras-go/content/oci/storage.go

Lines 109 to 117 in 0f1dc30

	// Delete removes the target from the system.
	func (s *Storage) Delete(ctx context.Context, target ocispec.Descriptor) error {
	path, err := blobPath(target.Digest)
	if err != nil {
	return fmt.Errorf("%s: %s: %w", target.Digest, target.MediaType, errdef.ErrInvalidDigest)
	}
	targetPath := filepath.Join(s.root, path)
	return os.Remove(targetPath)
	}

[wangxiaoxuan273]

I'd like to work on this.