Description

Currently, OCI exporter fails when the image is multi-arch and foreign arch blobs are not present on the daemon content store.

We should allow skipping exportation of inexistent blobs.

Steps to reproduce the issue:

1. sudo ctr images pull docker.io/library/alpine:latest (without --all-platforms)
2. sudo ctr images export a.tar docker.io/library/alpine:latest

Describe the results you received:
ctr: content digest sha256:78f3ccd48cc6a55709b65c8fdb3ef81ed922c5393b064d63b0d35f51e0c9fb2e: not found

NOTE: This failure is not caught in CI, probably because other test case pulls the entire testImage with non-amd64 blobs

Describe the results you expected:
No error

Output of containerd --version:

containerd github.com/containerd/containerd v1.2.0-beta.1-18-g68979c92 68979c923a3a33d48735c358285cb0a506cc0665