fix(handlers): ignore tag not found errors when deleting a manifest

When deleting a manifest through the API, we 1) delete the manifest, 2) obtain the list of tags (if any) that point to that manifest and 3) delete them in a loop. If for some reason a tag is deleted between 2 and 3, the request will be halted with a 500 Internal Server Error. This is because we are not ignoring "path not found" errors during 3. This change fixes that.

The fact that we do not ignore these errors is generating a lot of 500 Internal Server Error responses which can be seen in Sentry, such as https://sentry.gitlab.net/gitlab/container-registry/issues/2804563/?environment=gprd.

Related to &6348.